My MacBook says the documents cannot be opened whenever I try watching a video or a movie

Hello AdontengAcheampong,

Thank you for using Apple Support Communities. Based on your post, you can't play videos on your MacBook. We'll do our best to assist.

For issues playing videos on your Mac, check out: If an audio or video file doesn't play on Mac

These also might be helpful for you:

Open and play a file in QuickTime Player on Mac

If QuickTime Player converts legacy media files before playing them

If the issue persists, are you trying to play the video within the QuickTime Player app or another app?

How did you access these files exactly? Are they downloaded or transferred to the computer?

Hoping this helps out, take care.