This error relates to the amount of attendance sessions a student has counted attendance codes for during the specified date range. (These can be found in the DfE census guidance).
Usually this will occur when an entry 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 an entry date of 31/08/2018 who started at the beginning of this 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 entry date under the core information heading, edit and save at the bottom of the page. The entry date ought to be the first day the student has attendance data at your school.