Why this problem appears when i try to buy something? (I have 50$ balance and i was trying to buy the $19.99 pack).

This is not iTunes Support.

Contact Support as directed.

Get Support -

https://getsupport.apple.com/

M1 Mac mini, macOS 11.4; Watch, series 5 GPS/cellular, watchOS 7.5; iPad Pro 2nd Gen, iPadOS 14.6; iPad Air 2, iPadOS 14.6; iPhone 12 Pro, iOS 14.6; TV 4th Gen, tvOS 14.6  HomePods, audioOS 14.6 and Airport Express w/ Airplay 2