Error 4200 arises to alert you to a staff member's date of birth being out of normal range. If you click on the Error, it takes you to the staff member's profile. Click on the cog next to their name to bring up the Personal Details page:
Check the accuracy of the date of birth, in particular the year of birth. Then click Submit at the base of the page to save the information. Return to the Census area to recalculate the census.