Class information is missing.
This error cannot be resolved until on or after census day.
On or after census day, generate your census. Once generated, this error should be resolved.
If it is not resolved, check if your school has scheduled classes correctly for primary age students. Make sure your timetable slots for your primary classes last the whole morning (until the start of lunch), and your afternoon session lasts the whole afternoon (until the end of the day) by following these instructions: Changing the Start and End Times of timetable slots in bulk
You can also change the time of the census to bring through different classes from the Classes section of your census: Can I change the time or date for our spring census?