During your free Shopify trial, if you'd like to add any paid app, you must enter a credit card number into Shopify if you haven't already. If you try to install the Custom Fields app during the free Shopify trial, you'll be taken to a page in Shopify asking you to enter your credit card information. Custom Fields has nothing to do with this process and you will not be charged for our app until the end of the Custom Fields trial period.
You can work around this by setting up your store as a Development Store instead. You'll have to start from scratch with your Shopify setup, but the Custom Fields app is designed to support Development Stores. This means that you can set up Custom Fields exactly how you want it and take as long as you want to set it up without being charged for your Shopify store, or by Custom Fields! To learn more, visit https://help.shopify.com/partners/getting-started/development-stores