Currently, in-game products can be purchased through App Store and Google Play Store. 
Due to differences in payment methods, we ask for your kind understanding that immediate assistance may be difficult.
 
  ■ For the payments that were declined or rejected:
  1. Force close the game and reenter in a stable network environment
  2. Check and troubleshoot the account & payment info on the CS Pages of Stores.
        Play Store (AOS) Customer Center  ｜  App Store (IOS) Customer Center
 
Also, this might be related to your carrier. Check payment limits & methods and consult with your carrier's customer service.

If the above methods do not resolve the issue, please the profile   → tap the Settings, and then send your inquiry through the [Opinions]
We will see what we can do.