There is a document here, from Microsoft. It describes the same error, but not the same behaviour.
It could be the problem however, so it's worth checking.https://support.microsoft.com/en-us/kb/822511
If this document doesn't apply, skip these steps.
Another possibility is the document is protected by track changes. See if you can accept or reject all changes, or turn off track changes, then save the document and try.
Failing this try closing Word, restarting your Mac, and try the operation again.
If this still fails, then it could be a problem with Fonts, and we can try rebuild Fonts for Word if these earlier steps didn't work.
Also, have you checked Software Update to make sure you are using the latest version of everything from Apple? Also, updated Word? Just click Check for Updates in the Help menu to do that.
Let me know if anything changes, or unusual happens, when you try each of these steps.
If unresolved by these steps we will try font caches etc, please leave the answer unrated as there's more to be done.
Thans for your patience,