This error specifically is when the Business App user hasn't been associated with a Google user in the Google Admin Portal.
1. Open Google Workspace in Partner Center
- This will take you to the Google Admin Portal, where you can see your users and their settings.
2. Next to the user that is having difficulty launching Google Workspace, click the three dots and select "Edit"
3. This will bring you to a screen where you can assign a Business App user. These are users that are on the account already and have access to the Business App. Please select the user that matches the email of the Google Workspace user.
4. Once this Business App user matches the email that is associated with the account, the user can launch Google Workspace through Business App > My Products.
If your client would prefer to log in directly through Google, instead of logging in through the Business App, you need to turn Single Sign-On OFF. In your Admin Portal, go to "Settings" on the left-hand side, then toggle off "SSO":
Please note when turning off SSO, a pop-up displaying what this means for your users will appear: