Ubuntu – Error processing package grub-efi-amd64-signed ubuntu 20.4

I recently had the exact same issue with Kubuntu 20.04. The problem is that the /boot partition has run out of spare space to update the kernel properly. Hence it repeatedly fails on each retry (i.e. sudo apt upgrade). You can check this by running df-H and if the Avail space is low (say under 100MB) for the /boot mount then you may run into the issue you are seeing. First try sudo apt clean and sudo apt autoremove then sudo apt upgrade to see if it automatically clears up space to fix the error. If not, see my solution below.

To fix it you need to free up the space on your /boot. How I did it:

1. Run sudo dpkg --list | egrep -i --color 'linux-image|linux-headers' to give you a list of installed kernels and kernel-headers in your /boot.
2. Now you need to uninstall all of the ones you dont want EXCEPT the current kernel (find out current kernel by running uname -r). You probably should KEEP any kernel newer than the current kernel too.
3. Uninstall each kernel and kernel-header you dont want using:

   sudo apt purge {kernel-package-name}
   

   For example I ran:

   sudo apt purge linux-image-5.4.0-26-generic
   ...
   sudo apt purge linux-headers-5.4.0-26-generic
   ...
   sudo apt purge linux-image-5.4.0-33-generic
   ...
   sudo apt purge linux-headers-5.4.0-33-generic
   ...
   And so on...
   

4. Once you have removed enough kernels to free up space, restart your PC then try sudo apt update and sudo apt upgrade again and see if it succeeds and the error message goes away.

In future when it fills up again you may have to repeat this. I always thought the old kernels should get removed automatically or by using sudo apt clean and/or sudo apt autoremove but it seems not the case on some installs.

If anyone knows how to auto-remove old kernels please share