Assuming there are no other issues, here’s how you can convert it to a private app

1. Enroll for ABM
2. When the enrollment and account is confirmed, secure the registered Organization ID and Organization Name
3. Create a new build
4. Change the App Distribution Method to Private
   (appstore > General > Pricing and Availability > App Distribution Methods)
5. Type in the registered Org ID and Org Name. Save.
6. Attach the new build
7. Resubmit