Why is it that at the time of entering a student's attendance I cannot find it in the list of students to be entered?

In the event that a student's attendance cannot be entered for a class in a collective course that he or she has taken, it means that the student's registration date for that course was incorrectly set later than the date of the class he or she actually took.

To correct a student's enrollment date for a course, navigate to Didactics → Group Courses and click the 4th icon Students enrolled of the course concerned.

At this point, click the icon Edit (first icon) of the student in question, enter the correct attendance start date, and click the Edit Subscription to save the operation.

At this point it will be possible to enter the student's attendance.