The error 'If Leaving Reason is 'Not Applicable - Change of Contract' then Destination must be the same' occurs when the reason for leaving was added for the employee when they were terminated, but the SWC Destination hasn't been added.
To resolve this:
Search for the employee and go to their profile.
Click Admin, then Edit an existing post/position.
Select the relevant position.
Select the Not Applicable - Change of Contract option from the SWC Destination dropdown.
Click Save.