Unsupported Video Format

It is also important to know the version of macOS being used on the computer since older versions of macOS may not support newer video formats used by later versions of iOS. Did you try clicking the "More Info..." button shown in the picture to see if that provides you with any more details or suggestions?

I believe your iPhone may have an option to allow the video to be converted to an older more universal video format when transferred to a computer, but I'm not sure this works when transferring them through iCloud. The iPhone may also have an option to save the video in an older more universal format, but that will only affect new recordings. I know these options exist for pictures anyway.

Edit: On the computer you can try using the third party app VLC to play the video since VLC is able to play a lot of different formats.

Hi,

I try to import 1920X1080 movie from iPhone 12 to Photos.app via iCloud, though it is able to play.

We need more detail about your movie.