I just converted my garage lighting to use the TP Link HS200 WiFi smart switch, which works great in the Kassa app. However, the devices show up in SmartThings but I cannot use them from there. They always report a network /server issue, but work just fine from the Kassa app.
My goal was to use one of the SmartThings buttons I have to give myself a switch inside the house to turn the garage lights on and off, since right now you have to go out in the garage and fumble in the dark to find the switch (thank you builders).
Check the device in the kasa app and see if it needs a firmware update.
Also, how long has it been since you enable the integration? Some people have reported it took a couple of hours after the first integration before their devices showed up on the SmartThings side.