change this to
`stateTopic = "zigbee2mqtt/smartthings_plug/state"`
The stateTopic is passive - won’t command anything in the get topic - so you need to subscribe to the main state topic to successfully receive the state.
But also: you may be hitting the bug where Things files aren’t properly loaded by openHAB after a save. Restart your openHAB for a quick fix, or do this.