Welcome to Apple Support Communities! It looks like you're having an issue with your billing information, and this pop-up keeps showing on your iPhone. I'll be happy to help. First, take a look at our resource here, as there may be a billing issue with your account:
If your payment method is declined, or you have a billing issue with a previous purchase in the iTunes Store - Apple Support
If you're not seeing an issue with your billing details, then try signing out and back in with your Apple ID. While this article addresses another issue, it still has the steps we'll want to take:
What to do after you change your Apple ID or password
Specifically, use these steps for signing out then signing back in with your Apple ID:
If you're not able to sign in with your Apple ID here, you may need to reset your Apple ID password:
iPhone, iPad, and iPod touch
If you’re using iOS 10.3 or later, go to Settings > [your name] > iTunes & App Store. If you’re using iOS 10.2 or earlier, go to Settings > iTunes & App Store.
If you're signed in with your previous Apple ID, tap your Apple ID, then tap Sign Out. Then enter your updated Apple ID and password.
If you forgot your Apple ID password - Apple Support