Payment required for app download on iPhone

You usually need a payment method -

You usually need a functioning payment method for your account. Even to download free apps, to re-download or to update apps.

You can find the acceptable payment methods for your country here;

Payment methods that you can use with your Apple Account - Apple Support

M1 Mac mini, macOS 15.2;  Watch, series 10 GPS-only, watchOS 11.2; iPhone 16, iOS 18.2.1; TV 4th Gen, tvOS 18.2; HomePods, audioOS 18.2 and Airport Express w/ Airplay 2

It tells you it’s a payment but really it’s free just check whether it’s £0.00 pounds or not. Hope this helped.