Why audio can import when video does not
A media container can carry separate video and audio streams. Premiere may be able to decode the audio stream while failing to decode the video stream, so the file is not necessarily “audio only” at the source.
Adobe’s current guide lists multiple causes, including deprecated QuickTime-era codecs and MOV/MTS sources that need closer inspection. It recommends identifying the source codec and format rather than relying on the filename.
Do not diagnose from the extension alone
An .mp4 file may contain H.264, HEVC, AV1 or another video stream. A .mov file can also carry many different codecs. The correct question is not “does Premiere support MP4?” but “does this Premiere environment support the video stream inside this MP4?”
When a separate edit copy is appropriate
If the video stream is unsupported in your installed Premiere environment, create a new edit-friendly file. Keep the original untouched so you retain the source quality and can revisit the workflow later.
If the source should be supported but still imports incorrectly, check Adobe’s known issues and test after updating before doing a quality-changing transcode.