The number of Part Time students recorded as not being in school has to be equal to, or less than, the total number of Part Time students on roll.
On the front page of the Spring Census, there is a Spring [year] Census Specific Data area, one of which has a link to Class Details. Here you will record who is in which class on Census day, as well as the number of students not in school due to being part time.
If the number of part time students not in school is greater than the number of part time students on roll, this error will arise. 
To check the figure entered as not in school, click on Class Details on the Census front page. Compare this to the figure calculated by going to Reporting > Custom Report> (new report). On the right side under the Features tab you will find Part Time as an option to select. Run the report.
Place a Y in the Part Time column filter, then press return. The figure at the base of the table will indicate how many students you have said are Part Time in your school.
