Google Tag Manager Integration not working properly

[DuaneBrown]

Those store would be on Shopify Plus plan as that is the only one you can change the checkout.

[CiprianSo]

well, they are not... 

do you know of any official statement from Shopify saying they don't support adding additional (GA) scripts on the Checkout page?

[s-sana]

There's not an official statement but it does require to make it enable from shopify support.

https://help.shopify.com/en/questions#/contact/chat

Explain your issue to them and ask them to enable GA unsafe script on checkout page