You will receive this error if a member of staff has is recorded as a higher level teaching assistant and is under 18 years of age.
It could be that an error was made when recording either the date of birth or the HLTA. To resolve this click the error and this will take you to the staff member's profile. Then click the cog next to the personal details, in this area check the date of birth is correct:
and the HLTA has been recorded correctly:
Ensure to click submit after any changes have been made.
Once this has been amended return to the Workforce Census area to recalculate the census.