Had a customer who was having an issue adding a custom dictionary to the Microsoft Word. I found the following article that walked me through what to do.

https://support.office.com/en-us/article/Add-or-edit-words-in-a-spell-check-dictionary-56e5c373-29f8-4d11-baf6-87151725c0dc#__toc306779609

Start by clicking File > Options > Proofing.

Add the custom dictionary.

We unchecked the RoamingCustom.dic. Clicked “Add”, and gave this new dictionary a name. Word asked if we were sure we wanted to add this new one. We clicked okay, and everything was working again.

I hope this helps someone else out there…