Hello lauratherose,
Thanks for that info and choosing the Apple Support Communities. If I understand correctly, you are unable to export videos from QuickTime on your MacBook Pro, as you get an error message. You can figure out if unexpected behavior is related to a user file or setting by trying to reproduce the issue from another user account. This process includes creating a new user account, logging in to it, and testing for the issue.
How to test an issue in another user account on your Mac
Next, try running First Aid on your hard drive with the built in Disk Utility tool, as this can correct simple issues like you're experiencing.
Disk Utility for Mac: Repair a disk using Disk Utility
Cheers!
