Comment 1 for bug 1866425

It also happens here.

user@laptop:/$ sudo apt upgrade
Reading package lists... Done
Building dependency tree
Reading state information... Done
Calculating upgrade... Done
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
1 not fully installed or removed.
After this operation, 0 B of additional disk space will be used.
Do you want to continue? [Y/n] y
Setting up initramfs-tools (0.136ubuntu1) ...
update-initramfs: deferring update (trigger activated)
Processing triggers for initramfs-tools (0.136ubuntu1) ...
update-initramfs: Generating /boot/initrd.img-5.4.0-14-generic
cp: cannot create regular file '/var/tmp/mkinitramfs_fmVQNe/usr/share/plymouth/themes/spinner/watermark.png': No such file or directory
E: /usr/share/initramfs-tools/hooks/plymouth failed with return 1.
update-initramfs: failed for /boot/initrd.img-5.4.0-14-generic with 1.
dpkg: error processing package initramfs-tools (--configure):
 installed initramfs-tools package post-installation script subprocess returned error exit status 1
Errors were encountered while processing:
 initramfs-tools
E: Sub-process /usr/bin/dpkg returned an error code (1)
user@laptop:/$