If your Shopify account does not have the Plus plan, adding GTM could be quite tricky. The article describes how to do that.
The official Shopify documentation guide covers adding GTM on Shopify Plus stores only:

There are two key elements/areas to cover when adding the code to Shopify:
1. ‘theme.liquid’ (which is essentially all non-checkout pages): you can access and modify ‘theme.liquid’ even if you have a non-plus Shopify account. Follow the instructions from the Shopify guide to add GTM to ‘theme.liquid’.
2. ‘checkout.liquid’ (which controls the code on checkout pages): it can only be accessed by Shopify Plus account users. The challenge is how to add GTM container to the checkout pages if you do not have the Plus account?
The not so good news is that you cannot add GTM container to checkout pages (if you do not have Shopify Plus). The good news is that GTM can be added to the order confirmation page.
Here is the solution on adding GTM container to Shopify order confirmation page (for non-Plus account holders):
Step 1: Navigate to Settings (left-hand-side navigation menu) -> Checkout -> Order Processing:

Step 2: Copy and paste your GTM container snippet into ‘Additional scripts’ section:

Step 3: Save the settings and run the debug to ensure the container is working properly.
Hope you have found this article helpful. If you have any questions or find any other solutions, let us know.

Author: Alex Zakharych