When I buy from within the applications I receive the message (your purchase could not be completed) Please help with that

If it also says to contact iTunes Support you should do as instructed 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’)