When this Query arises it is stating that no students have been recorded as leaving in the term before the census you are running. This is related to the attendance data being collected for that census, for both on and off roll students.
Note: Check the Census documentation for that Census to note the dates in scope for attendance.
For example, the Spring Census looks at attendance data for on roll and off roll students, from 01/08-31/12 (Autumn Term). If no students are recorded as off roll in this period, this Query will arise.
Go to Admin > Students > Search Not On Roll to note the leaving date for your students (the students at the top of the table will be the last ones to leave).
If it is correct that no students have left in the period the census is looking at, just annotate the file when you upload to Collect. If there are student leavers recorded, check their age; for this query, the census is reporting on students aged between 5 and 15 as at 31st August.