The could not open lock error means that you have another application open that handles installations and uninstalls.
You can only run one installation or removal at a time. The system puts a lock on the package management while any package management process is running.
Close any Open terminal, Synaptic Package Manager or Software Store in order to resolve that.
That will release the lock. In future, be sure to be mindful that you keep installations individually run.
The focal release: does not have a release file means that the repository you added does not have a package released for Focal repository. It is a non-functioning repository for you and should be remove.
You can launch Software & Updates then navigate to the other software tab. Scroll down the list to the avidemus repository and click on it to select it.
Then click the Remove button at the bottom of the window. That will resolve that issue.