Thanks again for all of your help. I have a question about what you wrote in your latest email.

You wrote:

You need to take the kdenlive ppa out of your apt config. Adding Ubuntu
repositories to your Debian is a good way to break your system.

...

Remove the ppa first and try again.

I looked around, especially in the etc folder. I looked for places indicated on https://www.debian.org/doc/manuals/aptitude/ch02s05s05.en.html

I have not yet found where my apt config is located.

Please help.

LeRoy