This error arises when no teachers or support teachers have been applied to the class in the Class Information area. Go to the front page of the Spring Census and click on Class Details:
Check the entry for the Class Name provided in the error (in this case, Orange Class). Look at the number of Teachers and Support Staff Present.
Click on the pencil icon to edit this entry and ensure at least one member of staff is applied to this class. Save the changes by clicking on the tick in the Actions column, and Save at the base of the page. Recalculate the Census.