apigee policy status disabled for /callback endpoint

I am working on /callback endpoint of a apigee proxy.

In one of the apigee policy i am trying to save the value of state which is generated in /authorize endpoint.

I am using assign message policy for ths.

<Assignvariable>
   <Name> callbackstate </Name>
   <Ref> request.queryparm.state </Ref>
 </Assignvariable>

But when i hit this /callback endpoint this policy status shows "Disabled" and its not saving in callbackstate. I am not able to understand why its showing disabled ?