Well, that sounds pretty similar to Internal error / system update
You installed some questionnable quality app (idk which one, it’s not impossible that it was level 7 'cause we don’t really flag apps with custom repo as bad I think …) and now it broke your setup 
So basically, I would :
sudo rm /etc/apt/sources.list.d/non-free.list-
grep -nr "sury" /etc/apt/sources.list*to find which line the ‘sury’ repo is configured, edit the file, and comment the line by adding a#in the front of it…