Unable to run the design optimization

I am trying to optimize some parameters of a Simulink block using "Design Optimization Toolbox". When I am clicking to optimize I am getting the following error:

The 'SubPath' property of the Simulink.SimulationData.BlockPath object must a string corresponding to an object within the block.  For example, if this block is a Stateflow chart, the 'SubPath' property must be a string referring to a Stateflow chart signal.

Where can I edit/fix that?