If you organization is using the Availability Locking feature, then they may be limiting your permission to edit your Availability Calendar once you've received an assignment. It is up to your organization (i.e. your scheduler) to decide whether they want to allow you access to change your availability after receiving an assignment. Each organization can choose whether or not to turn this feature on or off. You would need to contact your organization for further information on which decision they've made and why.
You can read more about the Availability Locking feature at this support article.