There are 3 main causes of this issue.
Having tracked changes applied to your citations (to fix this you just need to accept or reject all tracked changes in the citations and stop tracking before using the format function)
In some cases composite images (images containing other images) can prevent the add-in from functioning correctly. To fix this, right-click (control-click for Windows, Command-click for Mac) the composite image and select 'save as image' which will condense the composite into 1 image file, then just replace the composite with the single image file you saved.
It may be a compatibility problem - these are usually specific to Windows. To confirm Office for Windows compatibility you'll need to check your Office version number, which you can find by searching Windows for 'Add or remove programs' and clicking on the result:
Then scroll down to where Office is listed, click on the listing and make a note of the version number that appears:
The minimum Office version we support is 16.0.11629, so you may need to update Office to get the addin working.