Why my member is not able to update the standby in availability.

Standby option is driven by the priority set in the Masters.

Any priority, which is set to default; the system will make the Standby unavailable/greyed out for the Members in the system and Member App who are on the same or above the default priority.

Lets understand it here, as you see the default priority set here is - 4.

So, below Member will not be able to update the standby availability in the system.

And it will look like this in the Availability Section in the system.

Hence, to make this Standby option active in the system and Member app for this Member, you would need to set the priority to below 4, which is the default priority set for the Member in the Masters.