Hello Hirama666
Maybe you did select a period/day for which the student has no courses scheduled.
For example:
Student 1 has only 1 course which happens on period "Morning".
You add an absence for Student 1 on period "Afternoon".
The result is no absences are added.
Another example:
Student 1 has only 1 course which happens on "Monday".
You add an absence for Student 1 on "2023-10-20" which is a Friday.
The result is no absences are added.