I had similar problem when upgrading to Ubuntu 19.10 from 19.04 root on zfs.

sudo dpkg --configure -a
Setting up initramfs-tools (0.133ubuntu10) ...
update-initramfs: deferring update (trigger activated)
Setting up linux-image-5.3.0-24-generic (5.3.0-24.26) ...
Setting up friendly-recovery (0.2.41) ...
Sourcing file `/etc/default/grub'
Sourcing file `/etc/default/grub.d/init-select.cfg'
Generating grub configuration file ...
Found linux image: vmlinuz-5.0.0-25-generic in rpool/ROOT/ubuntu
Found initrd image: initrd.img-5.0.0-25-generic in rpool/ROOT/ubuntu
Found linux image: vmlinuz-5.0.0-36-generic in rpool/ROOT/ubuntu
Found initrd image: initrd.img-5.0.0-36-generic in rpool/ROOT/ubuntu
Found linux image: vmlinuz-5.0.0-37-generic in rpool/ROOT/ubuntu
Found initrd image: initrd.img-5.0.0-37-generic in rpool/ROOT/ubuntu
Found linux image: vmlinuz-5.3.0-24-generic in rpool/ROOT/ubuntu
Found initrd image: initrd.img-5.3.0-24-generic in rpool/ROOT/ubuntu
Warning: Failed to find a valid directory 'etc' for dataset 'rpool/ROOT/ubuntu@install'. Ignoring
Warning: Ignoring rpool/ROOT/ubuntu@install
Warning: Failed to find a valid directory 'etc' for dataset 'rpool/ROOT/ubuntu@1904'. Ignoring
Warning: Ignoring rpool/ROOT/ubuntu@1904
Warning: Failed to find a valid directory 'etc' for dataset '<email address hidden>'. Ignoring
Warning: Ignoring <email address hidden>
Warning: Failed to find a valid directory 'etc' for dataset 'rpool/ROOT/ubuntu@21Aug19'. Ignoring
Warning: Ignoring rpool/ROOT/ubuntu@21Aug19
Warning: Failed to find a valid directory 'etc' for dataset 'rpool/ROOT/ubuntu@mysql'. Ignoring
Warning: Ignoring rpool/ROOT/ubuntu@mysql
Warning: Failed to find a valid directory 'etc' for dataset '<email address hidden>'. Ignoring
Warning: Ignoring <email address hidden>
Warning: Failed to find a valid directory 'etc' for dataset '<email address hidden>'. Ignoring
Warning: Ignoring <email address hidden>
Warning: Failed to find a valid directory 'etc' for dataset 'rpool/ROOT/ubuntu@beforeupgrade'. Ignoring
Warning: Ignoring rpool/ROOT/ubuntu@beforeupgrade
device-mapper: reload ioctl on osprober-linux-nvme0n1p3  failed: Device or resource busy
Command failed.
device-mapper: reload ioctl on osprober-linux-nvme1n1p3  failed: Device or resource busy
Command failed.
Adding boot menu entry for EFI firmware configuration
error: syntax error.
error: Incorrect command.
error: syntax error.
Syntax error at line 162
Syntax errors are detected in generated GRUB config file.
Ensure that there are no errors in /etc/default/grub
and /etc/grub.d/* files or please file a bug report with
/boot/grub/grub.cfg.new file attached.
dpkg: error processing package friendly-recovery (--configure):
 installed friendly-recovery package post-installation script subprocess returned error exit status 1
Setting up grub-efi-amd64 (2.04-1ubuntu12.1) ...
Installing for x86_64-efi platform.
Installation finished. No error reported.
Sourcing file `/etc/default/grub'
Sourcing file `/etc/default/grub.d/init-select.cfg'
Generating grub configuration file ...
Found linux image: vmlinuz-5.0.0-25-generic in rpool/ROOT/ubuntu
Found initrd image: initrd.img-5.0.0-25-generic in rpool/ROOT/ubuntu
Found linux image: vmlinuz-5.0.0-36-generic in rpool/ROOT/ubuntu
Found initrd image: initrd.img-5.0.0-36-generic in rpool/ROOT/ubuntu
Found linux image: vmlinuz-5.0.0-37-generic in rpool/ROOT/ubuntu
Found initrd image: initrd.img-5.0.0-37-generic in rpool/ROOT/ubuntu
Found linux image: vmlinuz-5.3.0-24-generic in rpool/ROOT/ubuntu
Found initrd image: initrd.img-5.3.0-24-generic in rpool/ROOT/ubuntu
Warning: Failed to find a valid directory 'etc' for dataset 'rpool/ROOT/ubuntu@install'. Ignoring
Warning: Ignoring rpool/ROOT/ubuntu@install
Warning: Failed to find a valid directory 'etc' for dataset 'rpool/ROOT/ubuntu@1904'. Ignoring
Warning: Ignoring rpool/ROOT/ubuntu@1904
Warning: Failed to find a valid directory 'etc' for dataset '<email address hidden>'. Ignoring
Warning: Ignoring <email address hidden>
Warning: Failed to find a valid directory 'etc' for dataset 'rpool/ROOT/ubuntu@21Aug19'. Ignoring
Warning: Ignoring rpool/ROOT/ubuntu@21Aug19
Warning: Failed to find a valid directory 'etc' for dataset 'rpool/ROOT/ubuntu@mysql'. Ignoring
Warning: Ignoring rpool/ROOT/ubuntu@mysql
Warning: Failed to find a valid directory 'etc' for dataset '<email address hidden>'. Ignoring
Warning: Ignoring <email address hidden>
Warning: Failed to find a valid directory 'etc' for dataset '<email address hidden>'. Ignoring
Warning: Ignoring <email address hidden>
Warning: Failed to find a valid directory 'etc' for dataset 'rpool/ROOT/ubuntu@beforeupgrade'. Ignoring
Warning: Ignoring rpool/ROOT/ubuntu@beforeupgrade
device-mapper: reload ioctl on osprober-linux-nvme0n1p3  failed: Device or resource busy
Command failed.
device-mapper: reload ioctl on osprober-linux-nvme1n1p3  failed: Device or resource busy
Command failed.
Adding boot menu entry for EFI firmware configuration
error: syntax error.
error: Incorrect command.
error: syntax error.
Syntax error at line 162
Syntax errors are detected in generated GRUB config file.
Ensure that there are no errors in /etc/default/grub
and /etc/grub.d/* files or please file a bug report with
/boot/grub/grub.cfg.new file attached.
dpkg: error processing package grub-efi-amd64 (--configure):
 installed grub-efi-amd64 package post-installation script subprocess returned error exit status 1
dpkg: dependency problems prevent configuration of grub-efi-amd64-signed:
 grub-efi-amd64-signed depends on grub-efi-amd64 | grub-pc; however:
  Package grub-efi-amd64 is not configured yet.
  Package grub-pc is not installed.

dpkg: error processing package grub-efi-amd64-signed (--configure):
 dependency problems - leaving unconfigured
dpkg: dependency problems prevent processing triggers for shim-signed:
 shim-signed depends on grub-efi-amd64-signed | grub-efi-arm64-signed; however:
  Package grub-efi-amd64-signed is not configured yet.
  Package grub-efi-arm64-signed is not installed.

dpkg: error processing package shim-signed (--configure):
 dependency problems - leaving triggers unprocessed
Processing triggers for initramfs-tools (0.133ubuntu10) ...
update-initramfs: Generating /boot/initrd.img-5.3.0-24-generic
Processing triggers for linux-image-5.3.0-24-generic (5.3.0-24.26) ...
/etc/kernel/postinst.d/dkms:
 * dkms: running auto installation service for kernel 5.3.0-24-generic
   ...done.
/etc/kernel/postinst.d/initramfs-tools:
update-initramfs: Generating /boot/initrd.img-5.3.0-24-generic
/etc/kernel/postinst.d/zz-update-grub:
Sourcing file `/etc/default/grub'
Sourcing file `/etc/default/grub.d/init-select.cfg'
Generating grub configuration file ...
Found linux image: vmlinuz-5.0.0-25-generic in rpool/ROOT/ubuntu
Found initrd image: initrd.img-5.0.0-25-generic in rpool/ROOT/ubuntu
Found linux image: vmlinuz-5.0.0-36-generic in rpool/ROOT/ubuntu
Found initrd image: initrd.img-5.0.0-36-generic in rpool/ROOT/ubuntu
Found linux image: vmlinuz-5.0.0-37-generic in rpool/ROOT/ubuntu
Found initrd image: initrd.img-5.0.0-37-generic in rpool/ROOT/ubuntu
Found linux image: vmlinuz-5.3.0-24-generic in rpool/ROOT/ubuntu
Found initrd image: initrd.img-5.3.0-24-generic in rpool/ROOT/ubuntu
Warning: Failed to find a valid directory 'etc' for dataset 'rpool/ROOT/ubuntu@install'. Ignoring
Warning: Ignoring rpool/ROOT/ubuntu@install
Warning: Failed to find a valid directory 'etc' for dataset 'rpool/ROOT/ubuntu@1904'. Ignoring
Warning: Ignoring rpool/ROOT/ubuntu@1904
Warning: Failed to find a valid directory 'etc' for dataset '<email address hidden>'. Ignoring
Warning: Ignoring <email address hidden>
Warning: Failed to find a valid directory 'etc' for dataset 'rpool/ROOT/ubuntu@21Aug19'. Ignoring
Warning: Ignoring rpool/ROOT/ubuntu@21Aug19
Warning: Failed to find a valid directory 'etc' for dataset 'rpool/ROOT/ubuntu@mysql'. Ignoring
Warning: Ignoring rpool/ROOT/ubuntu@mysql
Warning: Failed to find a valid directory 'etc' for dataset '<email address hidden>'. Ignoring
Warning: Ignoring <email address hidden>
Warning: Failed to find a valid directory 'etc' for dataset '<email address hidden>'. Ignoring
Warning: Ignoring <email address hidden>
Warning: Failed to find a valid directory 'etc' for dataset 'rpool/ROOT/ubuntu@beforeupgrade'. Ignoring
Warning: Ignoring rpool/ROOT/ubuntu@beforeupgrade
device-mapper: reload ioctl on osprober-linux-nvme0n1p3  failed: Device or resource busy
Command failed.
device-mapper: reload ioctl on osprober-linux-nvme1n1p3  failed: Device or resource busy
Command failed.
Adding boot menu entry for EFI firmware configuration
error: syntax error.
error: Incorrect command.
error: syntax error.
Syntax error at line 162
Syntax errors are detected in generated GRUB config file.
Ensure that there are no errors in /etc/default/grub
and /etc/grub.d/* files or please file a bug report with
/boot/grub/grub.cfg.new file attached.
run-parts: /etc/kernel/postinst.d/zz-update-grub exited with return code 1
dpkg: error processing package linux-image-5.3.0-24-generic (--configure):
 installed linux-image-5.3.0-24-generic package post-installation script subprocess returned error exit status 1
Errors were encountered while processing:
 friendly-recovery
 grub-efi-amd64
 grub-efi-amd64-signed
 shim-signed
 linux-image-5.3.0-24-generic