error message "Some files cannot be processed"
I ripped an old VHS tape to QuickTime. I've been doing this for months. Today when I tried to copy the a file to my network backup, I got this error message: "Some files cannot be processed, either because you don't have permission to modify them, or because they are on a locked volume." I've restarted my Macbook and still I get the error message. I checked "Get info" on the file and the "locked" box is not checked. It also looks like I have permission to read and write the file.
What does that error message mean and how do I fix it? Thank you.
MacBook Air 13″, macOS 10.15