This thread is about Word for Mac. You're using Google docs, if I understand correctly? If so, do run through the troubleshooting instructions and post to a new thread with any output (and note that Google docs integration is in beta and doesn't exist for Mendeley at all so switching wouldn't do you much good).
Upgrade Storage
Zotero experienced an error updating your document.
Would you like to view troubleshooting instructions?
in Google Docs, Chrome browser Version 78.0.3904.97 (Official Build) (64-bit), Mojave OS 10.14.6 (18G87)
Is it time to move to Mendeley, or you will fix this soon?
(and note that Google docs integration is in beta and doesn't exist for Mendeley at all so switching wouldn't do you much good).