App Store Setup
In order to add in-app purchases to your app on the App Store, you need to complete:
Apple's agreements for Paid Apps
Setup contacts
Enter your banking information
Submit taxes forms
You can find more information on all these steps in Apple's documentation.
Paid Apps Agreement Status
Log in to https://appstoreconnect.apple.com/ and select Agreements, Tax, and Banking. Under the Agreements tab, make sure that Paid Apps shows status Active. If it is not Active, follow the instructions to complete the agreements.