Hello Hirama666
The Periods listed on the Add Absences screen are the periods which have been associated to courses.
So for example, if you have not setup any course periods in a school, you won't have any periods to check.
Do you understand?
As an admin, you should take attendance using the Users > Take Attendance program.
To add absences for specific students on specific days and periods, you can use the Add Absences program, yes.
Finally, you can upload screenshots from here https://upload-image.rosariosis.org/