Unable to create a child cart template

[Ajay Malik]

I created a child template using this gudie https://docs.whmcs.com/Order_Form_Templates#Creating_a_Custom_Order_Form_Template

For example:

orderfroms

-child_theme

--theme.yaml

and in that file i used 

config:
parent: standard_cart

and applied it via whmcs as well.

I copied checkout.tpl from parent to make some changes but the changes are not visible on frontend.