Why do you have to create an Apple Business Manager Account?

Apple has updated the way Business need to distribute Apps. See https://developer.apple.com/videos/play/wwdc2019/304/

iRiS was using a very straightforward way to Distribute the Apps, using what Apple Call in-House Apps:

But this way is now no longer valid for Apple, therefore iRiS has decided to use Custom Apps:

Custom Apps requires you to create an Apple Business Manager Account. Please follow the steps here to create one: https://support.apple.com/en-gb/guide/apple-business-manager/apd402206497/web

If you have a DEP or VPP Account, you will need to upgrade it for free into a Apple Business Manager Account : https://support.apple.com/en-gb/HT208817

If you already have an Apple Business Manager Account, iRiS will need two things to Publish the App into that Account:

1. The Organization Name and ID of the Account> go to  https://business.apple.com > Settings > Enrollment Information:

From App Store connect iRiS will fill the Information given and then Push the App for Review to the Apple Team: 

2. Still from Apple Business Manager the second information you will need to allow is Custom Apps, from Apple Business Manager go to Settings > Enrollment Information and Enable Custom Apps.

You will only be able to enable it if you have an Administrator and if your Tax Information is correctly filled :

When iRiS will be able to publish the App into your Apple Business Manager Account, the App will be listed in Custom Apps as such:

If you already have an Apple Business Manager Account and your iRS App is published as a Custom App, there will be two ways to distribute the App: via your MDM (guide here for the MobileIron Process) or via Redemption Codes.