How do I complete my Apple ID to download apps?

You usually need a payment method -

Unless you have a NONE option in the payment methods listed in your account details, you usually need to have added a valid payment method to your account. Even to re-download or update apps and for free apps.

On a Mac;

Open the Music app

Choose the iTunes Store

Choose Account in the Music Quick Links on the right side of the store window

Sign in to your Apple ID

Add a payment method to your account details

On an iOS device;

Open the Settings app

Chose your Apple ID

Choose Payment & Shipping

Add a payment method to your account details

M1 Mac mini, macOS 26.2;  Watch, Series 10 GPS-only, watchOS 26.2; iPhone 16, iOS 26.2; TV 4th Gen, tvOS 26.2 HomePods, audioOS 26.2

If you are seeing this message as part of wanting to download something from one of the stores, see if what they mean is "verify payment". Add a payment method to your Apple ID as outlined in: Add a payment method to your Apple Account - Apple Support .

On an iPhone:

Open the Settings app.

Tap your name.

Tap Media & Purchases, then tap View Account. You might be asked to sign in with your Apple ID.

Tap Manage Payments.

Read the support article for how to do this on other platforms.

Sometimes it can also mean you need to accept the terms and conditions for store use. Scroll all the way to the bottom of a message saying this and accept them (ideally also read them first).

I also see you say you are using an iPhone 7 with iOS 15. You may not find all that many apps available for that iOS version anymore. If you want to run newer apps, you will have to buy newer equipment.