How to fix E: /boot has insufficient space for new kernel in APT

Check the current space usage of the /boot partition:

df -h /boot

If the 'Use%' column shows 100%, the partition is indeed full.

Find the kernel version you are currently running (do not remove this one):

uname -r

Example output: 5.15.0-91-generic. This is the kernel you must keep.

View all installed kernel packages:

dpkg -l | grep linux-image

This shows all kernel versions. Identify old ones (lower version numbers) that differ from your current running kernel.

Remove old kernel packages one at a time. Replace VERSION-NUMBER with the kernel version you want to remove:

sudo apt remove --purge linux-image-VERSION-NUMBER-generic
sudo apt remove --purge linux-image-unsigned-VERSION-NUMBER-generic

Example:

sudo apt remove --purge linux-image-5.13.0-44-generic
sudo apt remove --purge linux-image-unsigned-5.13.0-44-generic

Remove the 2-3 oldest kernels, keeping at least one older kernel as a backup in case the current kernel has issues.

After removing old kernels, run autoremove to clean up related dependencies:

sudo apt autoremove --purge

Then update the bootloader configuration:

sudo update-grub

Confirm that /boot partition now has free space:

df -h /boot

You should see the 'Use%' below 80%. You can now proceed with kernel updates.

If the partition is so full that apt cannot run, use dpkg directly:

sudo dpkg --configure -a
sudo dpkg --purge $(dpkg -l | grep linux-image | awk '{print $2}' | grep -v $(uname -r | sed 's/-[a-z]*$//'))
sudo apt-get -f install

This forcefully removes old kernel packages without relying on apt's dependency resolution.