Difficult to say why as you haven’t given any information. What happens when you try to buy it ? If you are getting an error message what does it say ? If you are getting a message to contact iTunes Support have you done so ?
Ok so I just talked the the people of my card and they said the app or “merchant” I am trying to buy from is reversing all attempts at a purchase! The app is “golf clash”. I just want to buy some gems. What do I do now???