Important: Only Shopify Plus merchants will see the checkout.liquid file. If you are using the Basic Shopify, Shopify or Advanced Shopify plans, this file is not available to you to edit or change. Shopify Plus customers can view and modify checkout.liquid. For more information Shopify Plus, check out Shopify's article.
For assistance modifying, editing or customizing checkout.liquid, you will either need to work with your Shopify Plus account manager or hire a developer.
Out of the Sandbox themes do not include checkout.liquid files and making changes to this file is considered an advanced customization.
There are a limited number of changes that all Shopify merchants can make to the checkout pages, which are outlined here.