verify my billing

The you have to verify your billing information before you can download any apps. The message seems pretty clear to me. See this Apple support link-->Change, add, or remove Apple ID payment methods - Apple Support.

Then there is something wrong with the billing information you entered. Here are some things that could be wrong:

1. Insufficient balance or account in arrears
2. Account suspended for suspected fraud (not necessarily by you)
3. Error in card number
4. Wrong expiration date
5. Wrong security (CVV) code
6. Address in your iTunes account does not match exactly the billing address for the card
7. The card was not issued in the country of the iTunes store

If checking all of these doesn't resolve it you will have to contact iTunes Store support-->iTunes Store - Contact Us - Official Apple Support