Do you mean GTag does NOT fire on Thank You page?
I have been working with GA and followed this instruction https://help.shopify.com/en/manual/reports-and-analytics/google-analytics/google-analytics-setup
It worked for me.
Just keep in mind you just need to insert UA-.... code into the Google Analytics account field.
Don't forget to enable "Enhanced Ecommerce" to track Conversion Report of GA.
I think you are talking about GTM code setup first check gtag code is it properly inserted in the backend of the website . It should be properly inserted in the head and body section of a website. After this, you can check in google tag manager in the preview section.
Once you will verify it you can track analytics and other tracking data for other queries you can contact me.