In that case you need to double check both the name of this item to make it is correctly spelled and capitalized and the actual type of the item to make sure that it is a Switch item sending the correct states. The configuration of the component and the syntax of the state expression are both correct. If you are not getting the correct behavior then your item is not returning the correct values.
Open the developer tools panel by pressing alt + shift + d, press the third icon in the button bar to go to the expression tester and paste your expression (=items.Z_way_switch_WallPlug3.state) there. If result doesn’t show ON or OFF then something is wrong with your item.