Meta Apps can only access Business Accounts they have explicit permissions for. Your MC Media manager probably is not linked to the same Meta App or does not have the right admin role assigned.
Go to your Meta App settings, check Connected Assets under WhatsApp, and verify MC Media is actually added there. You may need to request access or have the MC Media account admin grant permissions to your App ID.
Also confirm your permanent token was generated with the correct Business Account selected during creation.