I believe I experienced the same, with root cause a lack of free space in /boot. After the upgrade failed with errors, but _before_ attempting a reboot, my workaround was:
apt autoremove mkdir /boot_moved_20201011 mv initrd.img-4.15.0-118-generic /boot_moved_20201011/ apt-get upgrade mv /boot_moved_20201011/initrd.img-4.15.0-118-generic .
#reboot
I believe I experienced the same, with root cause a lack of free space in /boot. After the upgrade failed with errors, but _before_ attempting a reboot, my workaround was:
apt autoremove 20201011 img-4.15. 0-118-generic /boot_moved_ 20201011/ 20201011/ initrd. img-4.15. 0-118-generic .
mkdir /boot_moved_
mv initrd.
apt-get upgrade
mv /boot_moved_
#reboot