When your ubuntu distribution is installed using the LVM partition system, you can run into an issue while performing updates and upgrades of the distribution because, at some points, the /boot partition becomes too small. This results in messages of this type:

Not enough free disk space 

The upgrade has aborted. The upgrade needs a total of 617 M free 
space on disk '/boot'. Please free at least an additional 282 M of 
disk space on '/boot'. 

When this happens,

  • check the content of your /boot partition (with ls /boot) and remove all unused kernels. This can be performed with
    sudo apt autoremove --purge
    or, if you have old kernels and images, this sometimes require that you remove them one by one by removing the linux-headers-... and linux-images-... packages along with the remaining files in /boot;
  • if this is still not enough, edit (as sudo) the file /etc/initramfs-tools/initramfs.conf and change the line
    and run
    sudo update-initramfs -u -k all
    sudo apt autoremove --purge

This should be enough to solve the problem.