You will receive this error if a staff member's leaving reason is recorded as Not Applicable - Change of Contract and the destination on leaving is recorded as anything apart from Not Applicable - Change of Contract.
To resolve this locate the staff member in Admin > Personnel > Staff. On the staff profile click the cog next to the contract in question and in this area the leaving reason and destination on leaving can be seen on the left:
Once you have made any changes ensure to click update to save.