Hi powdercoat
You are correct about the reason for the fault message, in that, that form is only available for employees who use employee self-onboarding.
In your situation, if you go to view the pending invites (In AccountRight: Click Employee Self-onboarding -> Pending Invites -> Log into browser; in Browser: Navigate to the employee screen), you should see the invite there for your employee showing as "sent". You can click the 3 dots on the right and click"Cancel Invite" -> "OK" to then cancel the invite. Just be sure to click "Cancel" on the next popup which asks if you'd also like to delete the employee.
If you ever send an invite to the incorrect details in future, it's recommended to follow the above steps and cancel the existing invite, but choosing "Delete" to delete the existing card (Or alternatively, deleting the card directly in AccountRight or browser will automatically cancel the invite), and then simply invite the employee again with the correct email address.
Regards
Itai Etzman