This can happen when you are signed into multiple Google accounts or signed into a Google account that is not associated with your Matik login email. Sign out of all Google accounts not associated with Matik, and sign into your Matik Google account.
For more on Tagging with the Google add-on, see the Tagging Google Slides with the Matik Google Add-On article.
If you can't find the Google add-on in the menu bar
Google has changed the location of add-ons in the menu bar. The new add-on can be found by going to Extensions > Matik > Open Matik.