Hi there,
So there are two way’s to fix this. 1 is to add the Google Tag Manager to the checkout by simply going to Settings > Checkout > Additional scripts. This is the easy none technical way.
The second option is to add your GTM as a custom pixel. Instructions on how to do that you can find here: https://help.shopify.com/en/manual/promoting-marketing/pixels/custom-pixels/gtm-tutorial
Hope this helps!