The lecturer is unable to edit previously entered student attendance data, why?

It is necessary to verify that the Lecturer is enabled to change the attendance entered and whether he/she is within the maximum time to do so.

Navigating in Configuration → Settings → Settings related to Teachers, scroll down to the "Class attendance information can be changed only within  days after the first insertion (-1 no limit)." 

Then check that no more days have passed since the lesson than those entered in this box; if not, it is necessary to change the number of days within which the Lecturer can make changes to the attendance entered.