Can not map local user task variable to output variable


Hello dear camunda people,
I tried to map local variable of a userTask to a process task. I call rest method ‘Update/Delete Local Task Variables’ to create localOutput2 (I checked it created using Cockpit), then I call rest method Complete Task to complete this task, but get an error : RestException, Cannot complete task 57b079e3-a313-11e9-9805-0242ac120006: Unknown property used in expression: ${localOutput2}. Cause: Cannot resolve identifier ‘localOutput2’
what’s wrong ?
here is my task definition in the bpmn:

<bpmn:userTask id="UserTask1">
          <camunda:inputParameter name="localInput1">${global1}</camunda:inputParameter>
          <camunda:outputParameter name="global2">${localOutput2}</camunda:outputParameter>



thanks @thorben
Your comment on a Stack Overflow Question guided me. I Should set local variable in the Execution that executes that user task not in the user task. this subject is not explicitly described in the documentation