If you get the following Message when trying to Get Licenses for your Custom App in Apple Business Manager:
Please make sure the Apple Id you are logged inside Apple Business Manager is a Managed Apple Id: https://support.apple.com/en-gb/guide/apple-business-manager/mdm1c9622977/web
If you are logged in with a Managed Apple Id, please call Apple Business Support : https://support.apple.com/en-gb/business