Office add-ins cannot be installed when you're signed in to Office with your Brown Microsoft account. As long as your add-in does not require access to your Brown Microsoft account, you can follow these steps to sign out of Office before adding the add-in. 

1. First, make sure that all Word files are closed and the application is fully exited. Once you confirm this, open Word again.
2. When it's open, create an empty file.
3. Click on File (top-left) and then click on Account (left-hand side, towards the bottom).
4. You should see your account signed in (in the format of username@ad.brown.edu). Click on Sign out then click Yes at the prompt.
5. Close Word out completely.
6. Attempt to install the add-in again.