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

https://helpcenter.steinberg.de/hc/en-us/articles/115000808250-Video-support-in-Steinberg-products

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

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

Many thanks