Error importing videos to Photos in Catalina: Unsupported resource set: video,xmp
macOS 10.15 (Catalina) has removed support for a number of things, the old Quicktime Pro no longer works (unfortunate as it is a nice, simple, movie editor) and (among other things) support for older codec movie files.

It is likely that those files that won't import are in a (now) unsupported format.

I would recommend just re-encoding them into a modern format like H264. Handbrake is what I use for such tasks. it is free and open source, updated regularly and highly recommended.