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)

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

That warning requires you to follow this link.

My App Store purchase could not be completed

I can’t make purchase