I am not able to buy diamonds in game why?

As the screen shot shows, the iTunes Store can't process the purchase.

Contact iTunes Support for assistance. See the Apple Support articles below for contact information.   

iTunes Support – world wide 

Apple Support schedule phone call     

Billings and Subscriptions 

If necessary, try Twitter

https://twitter.com/AppleSupport.          People have posted it may take an hour or more to get a response.