Unity keeps giving me an "Invalid build path" error message.

I’ve been trying to build some scenes in this project for Mac OS X and WebGL. Every time I’ve tried anything but windows, it gives me an error message stating that what I was trying to put the build in was a “Invalid build path”. Can I have help?

I’ve actually found a fix to this. Export to the desktop. I guess Windows 10 just doesn’t like other OS’s files being in the documents folder.

More info:
I am using Unity 2018.3.0f2.
the error message read exactly:
Invalid build path: c:/users/myuser/documents
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)