Why won't QuickTime 10.5 open an MP4 file on my MacBook?
Why won't QuickTime 10.5 open an mp4 file on my MacBook (Sequoia 15.3.1)?
[Re-Titled by Moderator]
MacBook Pro 14″, macOS 15.3
Maybe the movie has some incompatible video or audio codec? MediaInfo or Invisor can display them so you might post those details.
3rd party apps like IINA can play almost any codec so use it or convert incompatible codecs.