This means that you have not entered a figure for the number of part time students not in school in the Class Information area. On the front page of the Spring Census, under Spring Census Specific Data, there is a Class Details button.
A table will come up where you need to enter how many students are in specific classes, and also record additional pupil data, such as part time students not in school (at the top of the table below):
Enter the figure here, Save, and then recalculate your census.