Why cant i download facebook?

The store requires a payment method that can be correctly charged to access the store services including downloading free apps and updating apps. If it stopped being verified for any reason (usually because it was unable to be charged for something), then it will require re-verification and will pop up the screen until its verified. If you have a pending purchase, it will need to be completed as part of the verification process.

If your payment method is getting declined, you'll need to contact your bank or credit card issuer to find out why.

Once the payment method has been correctly verified it can be removed, and it should not ask for it again.

See here: If your payment method is declined in the App Store or iTunes Store - Apple Support

Note that for verification, Apple may place an authorization hold on it. This hold is reversed shortly after verification is completed.

See here: About payment card authorization holds from the App Store or iTunes Store - Apple Support

Have you tried verifying your billing information?

Apple ID payment information - change

If necessary, contact iTunes Support. Either use the form or scroll to the bottom of the page in the 2nd link. Follow through the screens to get a chat or telephone option.  You may need to pick a problem that is slightly different than yours to get a chat option.  

iTunes Support

iTunes Support – world wide