Classroom Manager needs appropriate permissions to work properly. Even if the person using it is a Super Admin for the domain
Set Classroom data access
This article is for G Suite Administrators
As a G Suite administrator, you can choose whether or not users in your domain can grant access to their Classroom data to other applications via OAuth. For more information, read the Classroom API overview or view all of the Classroom API technical details at developers.google.com/classroom.
To set the Classroom data access policy:
Sign in to the Google Admin console.
Choose one:
If you’re using G Suite for Education or G Suite for Nonprofits, click AppsG SuiteClassroom.
If you’re using G Suite Basic, Business, or Enterprise, click AppsAdditional Google servicesClassroom.
Click Data access and next to Classroom API, check or uncheck the box to allow users to grant access to their Classroom data.
Note: You can configure access by domain and by organizational unit.
This information is copied from this page https://support.google.com/edu/classroom/answer/6250906