I've also had that happen when I've left the browser open too long and the system timed me out. If this is the case, log back in and try again. If that's not it and the job is still listed read the requirements again and verify that you are eligible. If you think you should be eligible and are still having difficulties, email the scheduler: They want to get it assigned and need to know if there is a technical issue.
Hard work builds character and homework is good for your soul.