While performing the searches getting the "usage limit exceed 500 MB" warning messages , To overcome this error , We increased the default srchdiskquota limit from 500MB to 1000MB for the specifice roles in authorize.conf.
But still not resolving the same error persists.
@gkumarashanmugam
Are you by any chance using the trial version ?
Also as @richgalloway had suggested, try restarting the splunk instance after modifying the authorize.conf.
Did you restart Splunk after modifying authorize.conf?