Based on this old thread :
To resolve this issue, you need to add the user account as an external user in the tenant. This can be done by following these steps:
- Log in to the Azure portal using an account that has the necessary permissions to add external users.
- Navigate to the Azure Active Directory blade.
- Click on the 'Users' blade.
- Click on the 'New guest user' button.
- Fill in the required details for the new user and invite them to the tenant.
- Once the user has accepted the invitation and their account has been created in the tenant, they should be able to access the application using their account.