In order for Shipped assets to be implemented in the checkout page, checkout.liquid theme file must be enabled. Use the following instructions to enable checkout.liquid theme file.
Check if you have "checkout.liquid" theme code enabled by going to Theme from the left menu bar, click on "Actions" dropdown and select "Edit code".
In the search field, enter "checkout.liquid".
If the search returns a result, then your "checkout.liquid" theme code is enabled.
If the search doesn't return any results, then you need to enable the "checkout.liquid" theme code by contacting Shopify Plus support (https://help.shopify.com/en/questions/plus-merchants#/login). Please note that the request must be from the store owner account.
During the request process, Shopify Plus support will require you to acknowledge that once checkout.liquid is enabled, any future updates to checkout will have to be done manually. Once you acknowledge, they will enable checkout.liquid to your store.