Hi I can't buy gems in clash royal

Hi

Check that your payment information is correct by following the instructions here (including being sure to check that your address details fully match those on file with your card issuer):

• Change or remove your Apple ID payment information - Apple Support
• Payment methods you can use in the iTunes Store, App Store, and iBooks Store - Apple Support

If you still need help, contact iTunes Support:

Contact - Official Apple Support

Do you have a valid credit card associated with your Apple ID?

Do you have restrictions set for In-App purchase?