Unsupported Apple Business Manager account
The account you used isn’t associated with an Apple Business Manager organization. If you have a legacy Volume Purchase Program account, that account must be invited to join an Apple Business Manager organization. For more information, see Upgrade from Apple Deployment Programs and the Volume Purchase Program and Invite VPP purchasers to Apple Business Manager in the Apple Business Manager User Guide.
If you can’t assign an app, see App-based assignment errors.