If a purchase has been disrupted it can be completed when restarting the app. In this case you will receive above error message with the request to complete the purchase.
If you have received a payment confirmation from the Google Play Store (you will receive payment confirmation from the iTunes Store at the end of the month), please check whether the credits have been transferred to your account (top left). If you have not received the payment confirmation e-mail the payment has not gone through.
In the unlikely event that you have received a payment confirmation but credits haven’t been added to your account please contact our Customer Support via the ‘Report Error’ form in Settings (small cogwheel symbol at the bottom centre of the page).