This is a quick guide on how you can enable/unlock and confirm access to the checkout.liquid for Shopify Plus Store.
Step 1: Go to your Theme Settings: Sales Channels > Online Store > Themes
Step 2:On the right side of the screen, click on 'Actions' and select 'Edit Code' - You'll then be redirected to the template files
Step 3: On the 'Layout' section, if you'll see the 'checkout.liquid' then you don't need to continue this guide, otherwise, please proceed and follow the instructions outlined in Step 4.
Step 4: Click on the 'Add a New Layout'
Step 5: The Add New Layout window will appear; Just navigate the options and select 'checkout' (Note: If you don't see the checkout in the option, you'll need to ask your Shopify Representative to enable the feature for you.)
Step 6: Once 'checkout' is selected, click on 'Create Layout'
Step 7: That's it! You should now see the 'checkout.liquid' in the layout list