Problems with h.264 video

I’m trying to import an h.264 video file but Cubase won’t do it - when I try to drag the .mp4 file into the video track I get a no-entry sign.

I’m on MacOs. I’ve checked that the framerate is compatible with Cubase - in fact I’ve tried various legal framerates and none of the work. I’ve checked the requirements at

I’ve also tried the same video in Apple Prores (422 and 4444) and used container as well, but no joy.

Has anybody got an idea how I can find out what the problem is?

Many thanks