How to edit video in Photos on Big Sur

What does the Info panel show for the videos that you cannot edit? It will depend on the frame rate and the codec shown in the Info, if Photos can edit the video.

Is your system version shown below your post correct? "macOS 10.15"? Then you need to upgrade to a newer system version to be able to use "adjust", "filters" and "crop" for a video. This ability has only come recently to Photos with the most recent version, Photos 6 on macOS 11 Big Sur. In Photos 5 on Catalina you cannot use the Edit panel on a video at all. You can only trim the video and set the key frame, and save a still frame.

This is not a referenced library, right? Does exporting original to the Desktop, and then editing it in QuickTime Player work?

Does it play in quicktime?

You might try re-saving in quicktime.

Where did this video come from?

It is not grayed out in my MP4's from a Canon.