This error concerns off roll students in the period that Census is looking at. The Census is only looking at students who are between 5 years and 15 years on 31st August prior to that academic year.
To check this error, you can use Reporting>Reports>Students (Blue Tab)>Admissions and Leavers Report. Enter the date range for the period in question and check for Leavers.
Or go to Admin>Students>Search Not On Roll and check to see if there are any student leavers the previous term. Check the age of any leavers, to see if any were less than 5 years old, and greater than 15 years, on the 31st August, as they will not be counted in the census calculations.
If you can confirm that no students in the age group in question have left the school in the term before this census, you need to check if you can annotate the Census file when you upload to Collect.