If you get Error 1496: No pupils no longer on roll have been recorded flagging in your census, you'll need to check you off-rolled your leavers at the end of last year.
Check your leavers from Students > Browse Students > Leavers.
If you do have students who left during the last term but this query is still flagging, check the student's ages.
If they are not statutory school age, they won't be counting towards the census' figures.
If you haven't had any other students leave your school during the last term, they won't be included in your census (for example no students left your school during Summer term when reporting the Autumn census).
You'll need to make sure you off roll all your leavers following Step 2 of the New School Year Setup process.