I've installed GTM in my Shopify website https://myecoganesha.com/
But except checkout page, it's working fine on Home Page, Collection Page & Products page.
On "Checkout Page" I checked GTM tag script through Tag Assistant (Chrome Extension) by Google, there is no issue But tag is not firing on "Checkout page".
I connected with Shopify support team, they are saying "our feature is for Shopify Plus we would have no documentation on how to do it."
Please help to fix this issue for "Non-Shopify plus"
Hope this will help you https://intellitonic.com/blog/add-google-tag-manager-shopify-checkout/
Checkout page can not be edited with simple plan. So Shopify plus is the solution :(