Double click on the error message (or right click and “Find in tree”). This will lead you to the “response” in the containment tree. Then use “Goto/Usage in Diagram”. If “None” is displayed then this parameter node is not used in your activity diagram – it is a duplicate (probably by copying it). Use right click at the “response” object from the activity diagram and you will see the correct node. (see image “usageInDiagram.png).
Just delete the one which is not used.