Currently, in-game products can be purchased through App Store and Google Play Store,

■ For the payments that were declined or rejected:
1. Restart the game after exiting in a stable network environment
2. Trouble with the account info on the 'Main CS Page;:
- Play Store (AOS) Customer Center: https://support.google.com/googleplay/?hl=en#topic=3364264
- App Store (IOS) Customer Center https://expresslane.apple.com/GetproductgroupList.action

Also, there might have been a telecommunication company problem: Check the payment limit, and method and check it through the telecommunication company's Customer Center.