My App Store purchase could not be completed.

If you can't make purchases or update apps, including free apps, you need to update your payment method or add a new one

See also the following Apple articles:

Add money to your Apple ID balance

Add money to your Apple ID balance – Apple Support (UK)

Payment methods you can use with your Apple ID

Payment methods you can use with your Apple ID – Apple Support (UK)

That warning requires you to follow this link.

If you can't make purchases or update apps, including free apps, you need to update your payment method or add a new one. 

My App Store purchase could not be completed

I can’t make purchase