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. 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.