If you've installed and tested the GTM Suite but are still not seeing the add to cart events fire on your product page then follow these steps to fix:
Ensure you have this tag in your GTM container:
If you don't have this then download the GTM container here.
Once you've downloaded then import this into your own GTM account and be sure you select MERGE and not overwrite.
Your next step is to put GTM into preview mode so you can browse your site to see the GTM console like shown below.
Navigation to your product page > click add to cart > and you should see a Click event on the left like this:
If you have a Click Class, Click Element, Click ID or Click Text then go back to GTM and update the Trigger to match the variable detail from the add to cart click.
In my example above I'm using the Click Classes so I've updated my trigger to look like this:
One you save then you are good to go.
Inside of GTM click "Refresh" on your preview mode > then refresh your product page browser of your website. Once it reloads then click add to cart again and you should see the add to cart tags fire successfully.