What is the "Using application 'Koofr' is currently not supported for your organization (your-domain) because it is in an unmanaged state" error?

Koofr is using the newest Microsoft Graph API which enables users to connect their personal OneDrive, OneDrive for business and OneDrive for education through the same interface. If your educational institution hasn't verified its domain with Microsoft, this method will not work and you will get this error. An administrator needs to claim ownership of the domain and modify the domain to Microsoft requirements. You will not be able to connect Koofr to OneDrive before that.