To make in-game purchases, you need to connect your account via Google Play Games settings.
Also, check that you have a reliable Internet connection.
Check your balance and connected payment methods in Google Play Store settings.
Find more information about answers to all your questions here:
https://support.google.com/googleplay/answer/1050566
https://support.google.com/googleplay/answer/1267137
If these tips didn't help, please get in touch with our technical support team, and we'll be sure to help you.