word 2016 can't save document
Sounds like you haven't granted Word permission to access your ~/Documents folder. Check in System Preferences > Security &amp; Privacy > Privacy tab and see if Microsoft Word is listed in the Files and Folders section with any unchecked items. You can also try adding it to the Full Disk Access area.

If that doesn't work, you might be looking at a more general permissions problem with your Home folder. In that case https://support.apple.com/en-us/HT203538 is worth checking out. Good luck!