These could be the reasons that you can't buy coins.
• A stable internet connection is needed. It's recommended to only buy coins when you have a good internet connection.
<iOS>
• All in-app purchases are made in the App Store. There could be a temporary App Store issue that is preventing in-app purchases. Try restarting your device.
• Try checking if there are any restrictions on your account. To check your restrictions settings, open [Settings], tap [General] and then tap [Restrictions]. Make sure that iTunes store and in-app purchases are allowed. For more details, click the link below.
Prevent in-app purchases in the App Store
• Your payment information may be incorrect. If your payment information isn't up-to-date, you can't make in-app purchases. Additionally, your payment method may not be supported by the store. To see the payment methods available for the App Store, click this link.
Payment methods that you can use with your Apple ID
• If the issue continues, please contact the Critter Crew team or Apple Customer Support. To contact Apple Customer Support, click this link.
Contact Apple Support
<Android>
• All in-app purchases are made in the Google Play Store. There may be a temporary Google Play Store issue that is preventing.
• Your payment information may be incorrect. If your payment information isn't up-to-date, you can't make in-app purchases. Additionally, your payment method may not be supported by the store. To see the payment methods available for the Google Play Store, click this link.
Accepted payment methods on Google Play
• If the problem with your purchase persists, please contact Critter Crew.