Today I have startet some release upgrades for some Ubuntu 16.04 VM machines. Sometimes I am strungling with this error.

error processing archive /var/cache/apt/archives/lxd_1%3a0.9_all.deb (--unpack):

apt purge lxd lxd-client

Attention if you need snap you should reinstall it

Found here

https://askubuntu.com/questions/1280499/upgrade-from-18-04-lts-to-20-04-lts-fails