After you complete the Work Party exercise you will need to supply an email address each time you create a new Onboarding case otherwise you will receive this error. As a thought you might want to add this to a Data Transform, perhaps the SetFieldDefaults data transform and have the email address filled in automatically. This should resolve your issue.
It is strange as you would need one email address for each Onboarding case. Just because you filled out the email address on one case doesn't create the object on another case. If you were to add the following line to your SetFieldDefaults data transform: