Please go to Time Off and Availability Settings and enable future availability / weekly availability setting.
Once you have enabled future availability option, employees can set their normal weekly availability under the "Availability Preferences" on their profile. This weekly availability specifies their regular availability. For example if an employee is only available Monday through Friday, they can block out the weekend.
The future availability is set under "My Schedule" and allows the employee to block out dates in the future, for example if they need a day off to visit the doctor.