When you are having issues with one or more Office applications, a full uninstall and re-install of the product is recommended. To do this, refer to the following steps.
Make sure that you are using the most up-to-date version of your Mac OS and you have done all of the updates.
Try to do a PRAM/NVRAM reset before doing this to see if that resolves the issue.
If not, delete all Office products using the following steps (you can also refer to Microsoft's support page on this).
1. Remove Office365/Office2016 for Mac applications
2. Open Finder > Applications
3. Command +click to select all of the Office 2016 for Mac Applications
4. Ctrl+click an application you selected and click Move to Trash
5. Remove files from your user Library folder To remove files from your user Library folder, you'll need to first set the Finder View options.
6. In Finder, press command+shift+h
7. On the Finder menu at the top, click View > As List, and then click View > Show View Options.
8. In the View Options dialog box, select Show Library Folder and save
9.Back in Finder, open Library > Containers and ctrl+click each of these folders and Move to Trash. Not all of these folders may be present.
Warning: Outlook data will be removed when you move the folders listed in this step to 'Trash'. You should back up these folders before you delete them.
10. Click the back arrow to go back to the Library folder and open Group Containers. Ctrl+click each of these folders if present, and 'Move' to 'Trash'.
11. Remove from dock and restart the machine.
12. If you put any of the Office applications in your dock, go to each one and ctrl + click > Options > Remove from Dock.
13. Restart your Mac to complete the uninstall.
To ensure all of the licence components for Office365 are gone, please follow the steps on this page to use the Microsoft Licence Removal tool. After you have done this, make sure to re-start your machine before Installing Office 365 again.
If Office still won't activate after doing all of these steps, download and run the Microsoft Licence Activation Tool .