If you are not able to add new Volunteers to Shifts or if you see a message as "Salesforce Field-set Config error" in the Job/Shift page, you should add fields to the add volunteer screen.
To add the fields using the fieldset,
- Log in to Salesforce as administrator, and click the Setup button. From the Object Manager go to Contact and search for Field sets. Click the edit button for the VolunteerSignupFS, and click the edit button. Drag and drop the fields to the add volunteer screen and Save it.
Once you make all these changes, you can add new volunteer to a Shift.