Comment 24 for bug 1874698

please install a higher version kernel like 5.4.0-28/29 first, boot with the new kernel, then sudo dpkg -r --force-all linux-image-unsigned-5.4.0-26-generic, install the v4 testing kernel to do the test.

I installed the v4 testing kernel on my machine, there is no errors:

hwang4@hwang4-Vostro-5390:/tmp$ sudo dpkg -i *.deb
[sudo] password for hwang4:
Selecting previously unselected package linux-image-unsigned-5.4.0-26-generic.
(Reading database ... 393690 files and directories currently installed.)
Preparing to unpack linux-image-unsigned-5.4.0-26-generic_5.4.0-26.30+skipv4_amd64.deb ...
Unpacking linux-image-unsigned-5.4.0-26-generic (5.4.0-26.30+skipv4) ...
Selecting previously unselected package linux-modules-5.4.0-26-generic.
Preparing to unpack linux-modules-5.4.0-26-generic_5.4.0-26.30+skipv4_amd64.deb ...
Unpacking linux-modules-5.4.0-26-generic (5.4.0-26.30+skipv4) ...
Selecting previously unselected package linux-modules-extra-5.4.0-26-generic.
Preparing to unpack linux-modules-extra-5.4.0-26-generic_5.4.0-26.30+skipv4_amd64.deb ...
Unpacking linux-modules-extra-5.4.0-26-generic (5.4.0-26.30+skipv4) ...
Setting up linux-modules-5.4.0-26-generic (5.4.0-26.30+skipv4) ...
Setting up linux-image-unsigned-5.4.0-26-generic (5.4.0-26.30+skipv4) ...
I: /vmlinuz.old is now a symlink to boot/vmlinuz-5.3.0-46-generic
I: /initrd.img.old is now a symlink to boot/initrd.img-5.3.0-46-generic
I: /vmlinuz is now a symlink to boot/vmlinuz-5.4.0-26-generic
I: /initrd.img is now a symlink to boot/initrd.img-5.4.0-26-generic
Setting up linux-modules-extra-5.4.0-26-generic (5.4.0-26.30+skipv4) ...
Processing triggers for linux-image-unsigned-5.4.0-26-generic (5.4.0-26.30+skipv4) ...
/etc/kernel/postinst.d/dkms:
 * dkms: running auto installation service for kernel 5.4.0-26-generic
Error! Your kernel headers for kernel 5.4.0-26-generic cannot be found.
Please install the linux-headers-5.4.0-26-generic package,
or use the --kernelsourcedir option to tell DKMS where it's located
Error! The dkms.conf for this module includes a BUILD_EXCLUSIVE directive which
does not match this kernel/arch. This indicates that it should not be built.
   ...done.
/etc/kernel/postinst.d/initramfs-tools:
update-initramfs: Generating /boot/initrd.img-5.4.0-26-generic
/etc/kernel/postinst.d/zz-update-grub:
Sourcing file `/etc/default/grub'
Generating grub configuration file ...
Found linux image: /boot/vmlinuz-5.4.0-26-generic
Found initrd image: /boot/initrd.img-5.4.0-26-generic
Found linux image: /boot/vmlinuz-5.3.0-46-generic
Found initrd image: /boot/initrd.img-5.3.0-46-generic
Found linux image: /boot/vmlinuz-5.0.0-1042-oem-osp1
Found initrd image: /boot/initrd.img-5.0.0-1042-oem-osp1
Found linux image: /boot/vmlinuz-5.0.0-1037-oem-osp1
Found initrd image: /boot/initrd.img-5.0.0-1037-oem-osp1
Found linux image: /boot/vmlinuz-5.0.0-1034-oem-osp1
Found initrd image: /boot/initrd.img-5.0.0-1034-oem-osp1
Adding boot menu entry for EFI firmware configuration
done