How do you know if there are licenses available on an Apple Business Manager or Apple School Manager?

1. In Apple Business Essentials, sign in with a user that has the role of Administrator or Content Manager.
2. Select Apps          in the sidebar, select an app, then select Licenses.

Besides viewing the number of assignments made for the app and seeing Collections they are assigned from, you can also see a list of specific users and devices that were not successfully licensed due to a lack of sufficient licenses. After additional licenses are acquired from the App Store, they automatically get applied to the users and devices listed.
Note: The apps listed on any individual user or device page reflect the apps that are assigned to that user or device through Collections. They don’t reflect which are successfully licensed. To check apps for licensing issues, check the Licenses tab for an app.