PURCHASE ERROR: The operation could not be completed. You are not permitted to make purchases.

This error notification indicates that the Apple ID details are incorrect. This could mean that you have downloaded the app with a different Apple ID to the Apple ID you are trying to purchase an issue or subscription with. If this is the case, delete the application from your iPad and download it again using the same Apple ID as you are using to purchase an issue or subscription. For instructions on how to delete an application, click here.

If your Apple ID details are correct then check to see if there are any restrictions on In-App Purchases on your iPad. Go to Settings > General > Restrictions > Allowed Content > In-App Purchases and make sure this toggle is switched to ON.