If you have the Sortd Gmail add-on (which you would have downloaded from the G-Suite marketplace), you should still install the Sortd Chrome Extension (which you can get from the Sortd Web site).

How do I know if I have the Sortd Gmail add-on but NOT the Chrome Extension?

You will see a Sortd icon in a bar on the right in Gmail that looks like this:

After you have installed the Sortd Chrome Extension, you will no longer see the Sortd icon on the right, but you will see a Sortd squirrel button on the left of your screen instead.  

Just a note that you can use the Sortd Gmail add-on without the Chrome Extension but the Chrome Extension is much more functional.  The Gmail add-on is however useful to also load is it gives you additional capabilities on mobile.