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Could you kindly let me know which document you have followed to achieve your requirement.
As per my knowledge only Shopify plus allows to make customization on the checkout page and not the other plans. So, can you please let me know if you are using the Shopify Plus plan.
It is possible but on cart page before checkout, what @Pallavi is referring is about the actual checkout page which you can not modify unless you have plus plan which costs 2K / month. If your business needs that kind of functionality to actually modify that page then you can do it.
In short what is mentioned in the tutorial is possible but that is on cart page just before clicking checkout button, hope this helps