If you plan to offer paid content within your app, it is necessary to define In App Products (In App Purchase Items) for each issue you plan to sell within your App. Those In App Products need to be setup within each App Store you use for distributing your app.
Please see the following guides for setting up In App Products in...
- Apple App Store
Create In App Products in Apple iTunes Connect
- Google Play Store
Create In App Products in your Google Play Account