iMovie will not import any mp4 or mkv files

macOS 14 Sonoma QuickTime Player does not play video codecs like H.265 with Codec ID hev1, VP8 and VP9, and audio codecs like Vorbis (Sonoma now supports Opus audio), and wrappers like .avi, .mkv and .webm.

ffmpeg can losslessly and very fast fix that H.265 with Codec ID hev1 issue with:

ffmpeg -i input.mp4 -c copy -tag:v hvc1 output.mp4

macOS 10.14 Mojave QuickTime Player does not play H.265 with Chroma subsampling 4:2:2 (Bit depth 10 bits).

IINA and VLC should have no issues with playing any decent new or old codec.

Handbrake, Shutter Encoder, VLC and ffmpeg can be used to convert old codecs to H.264 or H.265 video and AAC audio wrapped as .mp4 (or its very close cousin .m4v or .mov. I have used MacPorts to install ffmpeg but there are also precompiled versions).

You can check all those pesky codec details with apps like MediaInfo or Invisor.

Your file looks fine, and should be able to be opened with iMovie. You can try re-transcoding it with Handbrake.

Is it Sony XAVC-S by chance?

Can you put a small sample movie on some cloud site?