This error message can be due to blank approval steps and conditions. Here are the steps to resolve:
1. Check your approval steps. If they are blank click the reset button on each step and add your reviewers and conditions again.
Alternatively, the error message also appears in cases when the reviewer is picked from the Google Form response but not added as user in the Form Workflow Plus web app. Here are the steps to resolve:
1. Login to the web app at: https://app.formworkflowplus.com
2. Click the avatar menu > Account Settings > Manage Users or go directly via this link: https://app.formworkflowplus.com/configurations/users
3. Click Add User
4. Add the reviewer's email address and assign User role, as shown:
5. Click Create