This error will show when the figures entered in Admin > Census > Spring 2020 > Class details does not equal the number of students on roll.
To amend this go to Admin > Census > Spring 2020 > Class details (in the Spring 2020 Census Specific Data table).
In this area enter the information for the pupil reconciliation fields and each of your classes.
Once this is entered and equals the on roll number the message on the left above the save button will show in green.
Ensure to save once you have completed.
Also, This error can also be caused by duplicate SEN, overlapping dates in the SEN table and end dates overlapping. It can cause the error for Duplicate student records and Duplicate UPN. If you go to the student Profile>Support Tab > SEN white cog and check/edit these details, this will fix the error.
If the number of pupils on roll does not match your On Roll by Date figures, you may have hidden or guest pupils which excluded their data from the Census. You can check this in the Admin>Students>Group Updater area (Guest Pupil is under the student attribute of Enrol Status).