Your Apple iPad allows for the configuration of Restrictions to prevent the accidental buying of InApp purchases. This is a really good idea if you're giving the iPad to a child to play games which might offer extra features that can be bought inside the App.
If this setting is enabled via iPad Settings > General > Restrictions > In-App Purchases, you are unable to purchase an Annual Subscription or optional download from the Purchases page. You will see the following message displayed:
To change this setting to enable In-App Purchases, tap on the iPad Settings icon:
To enable In-App purchases, tap on iPad Settings > General > Restrictions and enable In-App Purchases.
In-App Purchases enabled:
Once you've purchased your Annual Subscription or other optional download from the Purchases page, disable InApp Purchases if you use this setting to prevent accidental purchases.