This error relates to the amount of attendance sessions a student has attendance codes for during the specified date range. (These can be found in the DfE census guidance).
Usually this will occur when a leaving date has been entered in a holiday which means it looks like the student was on role during the period that the census is looking at.
For example when spring census is looking at attendance between 01/08/2018 to 31/12/2018 and you have a student with a leaving date of 31/08/2018 who left at the end of the previous academic year it will result in this error. This is because the census is looking for counted attendance codes for this student between 01/08/2018 to 31/08/2018.
To resolve this error you would click fix error which will take you to the student's extended tab on their profile. On this page check the leaving date under the school information heading, edit and save at the bottom of the page. The leaving date ought to be the last day the student has attendance data, their last day in school.