There are several reasons why this may occur:
Some assignments are “self-assign”, which means you may assign the
shop to your account, which in turn removes the shop from the Job Board.
The mystery shop or audit might have already been assigned to someone
else. If this happens, after you submit your app, you will receive an
email notifying you that the audit was filled.
You might be choosing a date which is no longer available. Please
make sure you are choosing dates which are not grayed out on the
calendar.