convert 4k video to 1080p error

I’m trying to convert 4K videos on my iPhone 12 Pro to 1080p because they’re taking up too much space. I’ve tried adding to iMovie app, then changing the setting to 1080p and saving the video, but it just gives me an error saying it cannot be completed.

I also tried to use the shortcuts app, encode the video to 1080p and save to my media, but it gives me “An unknown error occurred (-12780) The operation could not be completed.”

Both my iPhone 12 Pro and iPad Air give the same error. Not sure what’s going on.