Ah.. Now I see your problem, must be becuase you are trying to execute this from a wrong level.
Please check the OS version:
#show switches --> If 6.x code, it will list out the Master and Locals, If 8.x Code will list out the MM and MDs
#show running-config | include master-ip --> Irrespective of the code, you will get the Master-IP, if you are on one of the Locals/MDs
Check the Master/MMs IP address, logon to the specific IP, then try again.
Or, you can go to the AAA profile of the Captive Portal Profile and try to change the User Idle Timer.
Or, you can change the Global TImer values, but remember this will affect the all other existing and upcoming AAA profiles.
With any of these above steps, this time it should definitely work.
If it is still not working, please share the screen-shot and outputs.
If possible, in the meantime, raise the TAC ticket as well.
Good Luck!!