When installing mega sync on Ubuntu 20.04.3 LTS
You are downloading MEGA version for wrong OS version. Ubuntu 21.04 and Ubuntu 20.04 LTS are not compatible in many libraries.
You have to explicitly specify that you need a package for Ubuntu 20.04 LTS:
on https://mega.io/sync
and then install megasync-xUbuntu_20.04_amd64.deb file.