In the weekly schedule settings you find the option, that applications from employees for shifts can be automatically accepted. This way you don't need to accept them yourself and the employees can directly plan themselves in the schedule.
However this function does not work if the employees have already reached their maximum working hours or have not stated a max. working hours in their profile.
You can edit this in each employee profile. Go to the section "employees", chose an employee and click on "edit". Now you can edit the max. working hours: