That’s really bizarre that the device doesn’t support the
off() methods which are officially defined by the SmartThings
I’ve reached out to the SmartThings team to see if they have any thoughts on it. From what I recall, this Kasa integration was part of SmartThings’ new CST program (Connects with SmartThings) where the hardware manufacturer can build their own integration and then self certify. My guess is something about this integration doesn’t play nice with SmartApps.
Do you use the SmartThings Classic mobile app and if so, can you turn these plugs on/off from that app? Or better yet, can you turn the plugs on/off from any other traditional SmartApps like ‘Smart Lighting’?
In the meantime, you could likely workaround this by using a community developed Kasa integration, but I believe most of those require keeping a PC or Raspberry Pi running a small Node.js script.