At the moment you can only select a group for the state condition. Selecting a light/plug instead is on the wish list for future development, see: Funktion if light is on or off
For now you can put your light/plug in a new zone/group and use that with the group state condition.
After you have created a zone/group for your plug only, on the screen of your smart control, tap the wizard button, tap Set up selected button, and tap Toggle group.
See also the following for more information about the smart controls: Device state automations (smart switches, motion sensors, lights, and plugs)
Let me know if you have any questions.