Yes and hopefully everything is resolved now. Long post but may be useful to others.
Simple question . look like this line added " in the files.
The /etc/apt/sources.list.d/mono-official-stable.list had an incorrect entry in it. The “ was removed from that entry and all is well.
Amazing help Pete. yes, this 123 post thread will come in handy to those of us who try to change to buster. An idea for future: consider using free teamviewer so you can do the stuff on the other guy’s computer as they watch.