If you have a pupil who is permanently excluded it isn't necessary to enter the amount of sessions that they were excluded for.
To amend this please go to the student profile via Admin > Students > Search Not on Roll and search the pupil name and choose. Here you will have the option to go to their profile, on the profile click onto the conduct tab and exclusions on the bottom left. Edit the permanent exclusion and change the sessions to zero and update. Once the census is recalculated this should remove the error.