Mike, OK I see what went wrong.
Here is an easy to follow set BUT honestly from my point of view, just using the static tar is easier for experimentation, for backout, and to keep multiple versions instead of installing the package as a system package.
The instructions you followed were for Ubuntu 14 at the beginning of the instructions. (Add "sudo" for the necessary commands below. The first step is correct that you already did and only have to do once:
apt install software-properties-common apt-transport-https
Then read down in the instructions to not use version 14: For Ubuntu 20.04 (Focal Fossa), run the third set of lines below: -- which in this case is:
Next edit the file /etc/apt/sources.list to make sure the added line contains [trusted=yes] as shown below:
Finally you run these:
apt update
apt install cin
BTW: there is now a Cinelerra YouTube channel with many expert examples of usage accessible from the website under Support:
BUT if you just want to make a quick video to see how to operate Cinelerra, you can follow the Quickstart.pdf file at: