When I issued the commend

sudo apt upgrade

I got the dreaded unmet dependencies error. I tried to fix it by running

sudo apt-get install --fix-broken

but just wound up in a loop. I finally fixed it by running

sudo apt-get clean 
sudo apt-get autoclean

to fix the corrupted system database.

I think the problem was due to the system being unable to find the nvidia sources and so it couldn’t find the dependencies.

Lovell Portrait