Why does it ask to verify if I am downloading a free app

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.

Apple ID payment information - change.  

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

When you tap on your Apple ID, a pop-up will appear on the center of the screen. Tap View Apple ID and enter your Apple ID password.

Then, scroll down and tap on Subscriptions. If any of your subscriptions are unpaid, your iPhone will say “Verification Required” when you try to download a new app.

To prevent problems like this from happening in the future, make sure the subscription automatically renews. A lot of subscriptions do automatically renew, like your Apple Music, Apple News, and streaming service subscriptions.