I’m trying to download an app that’s free and it’s asking for a payment

Is it asking for a payment or is it asking you to verify your billing information?

Log into your account in the App Store/iTunes and you should be able to change/verify it there. Or you can click the blue Edit payment methods in the article below. When signed in and the page loads, you will be on the credit card page. After updating that, click the Go Back button at the lower left and you will be on the address page.

Apple ID payment information - change.  

How to Fix “Verification Required” for Apps Downloads on iPhone and iPad