If you can't modify your availability it means your manager has locked it down.  Typically, an employee is given a time frame to fill in their availability and once that time frame is up managers lock it so it cannot be modified.  They do this because to ensure consistent staff availability while they create schedules for a session or a period of time.

You'll know when availability has been locked because it will say Closed to editing in the upper right hand corner of your My Availability page.