Error 4490 arises when a staff member with a Leadership Post (Head Teacher, Deputy Head or Assistant Head) does not have a Leadership Pay Range or Other Pay Range entered on their contract. Click on the Error link to take you to the staff member's profile, then click on the cog next to their contract to open it up:
Check the Pay Range entered in the contract, as well as the Post Held. If the Post Held is correct, alter the Pay Range from the dropdown to either 'Leadership' or 'Other'. Click on Update to save the change to the contract, then recalculate your Workforce Census.