You can request to call out of a shift if it is at least two weeks out. To do so, find the shift by going forward at least 14 days. Instead of the disabled "Clock-In" button, you will see "Call-Out".
Select the "Call-Out" button. You are required to explain why you can't work that shift. You will remain assigned to that shift unless the office approves the call out. You will receive a notification if your request is approved.