If your in-app purchase is disabled, you will see the message "was not able to load a product" and the get started button is disabled. 

To enable in-app purchases make sure you haven't restricted it previously

and make sure in-app purchase is switch to on. Apple has a FAQ here on this subject: https://support.apple.com/en-ca/HT204396

Also here is screenshot to show you where the in-app purchase switch is located.