Are you using two switches to control one same light? If the light just flicker, it could be caused by wrong wiring.

If you have 4 wires in one bundle (one will be red) and another switch this is probably a 3-way switch installation which is not supported by the HS200. You need to use the HS210 or do something custom with the wiring.

Part of this description is happening to me as well, specifically "After the installation the lights just flickered (fast)". I only have one switch, which I've swapped with a HS200, to control two outside lights. Everything seemed to go okay after installation. I got the switch on my WIFI network and could see it in my Kasa iPhone app. But when I try to turn the lights on outside, either through the app, or by manually pushing the switch, the outside lights flash on and off quickly. Then the switch seems to get into a bad state If I try to switch the light state again with the app, it can't connect to the switch, and the switch goes into a mode where the WIFI light flashes for a while. It looks like the switch is rebooting. Thoughts?

If it's not a loose wire then it's power through fixture wiring and the HS200 won't work there. There's enough juice to power the switch until you send power to the light. Then it loses power, resets the relay, and starts powering back up again.

I was able to fix my issue, which was due to problems with the wiring. In my situation, I was putting the HS200 switch into box that held 4 switches, i.e. I was replacing one of the 4 mechanical switches with the HS200 Smart Switch. In the box, there was a big bundle of white wires (perhaps 8 of them) with a big wire nut. In my erroneous installation, I arbitrarily pulled one wire off the bundle and connected it to the white wire of the HS200. I think this was not a good neutral wire. I fixed the problem by connecting the HS200 white wire (neutral wire) to the big bundle of white wires with the big wire nut (I also put back the white wire I had earlier pulled out from this bundle). That seems to have fixed the problem. I can now readily turn on and off the associated light with the switch (mechanically), with the Kasa app, or with our Google assistant through a voice command. ;)

It would be nice to have a trouble shooting section in the installation guide that specifically mentions the scenario where the light just flashes on then goes off when you try to turn it on, and suggest that is likely an issue with the neutral wire connection.

Yeah, they should have a troubleshooting guide. Bit tricky, there can be a few reasons for a missing neutral - power through fixture wiring is quite common, no pun intended ;), loose wires another. They may want to stop short of teaching basic electrical wiring (and code compliance) - if a bundle is too fat peel off one or two and jumper to a new bundle with the TP Link. Unhooking arbitrary wires is seldom a good idea... Good that you got it sorted!