I’m not able to pay for an app

This site is a user forum, not iTunes Support, we are fellow users, we can't help you. Go to the URL and contact iTunes Support : http://www.apple.com/support/itunes/ww (click on your country's flag, the country where you live, and which is therefore the country on the account ; click on Contact Support towards the top right ; click on Get Help under 'Talk To Us’)

Have you done as instructed and contacted iTunes Support via that URL ?