@Wayne-TP 

This is a convoluted process for many reasons. If you directly connect to the device before you get to the step where you say the device is ready, the Kasa app says you are not connected to the internet and refuses to proceed making you have to reconnect to the internet and repeat. You have to have your phone setup normally. Start the add device process, but when you get to the 'device is ready' sort of view, don't click on it. This is when you go to your WiFi settings and connect to the device and then go back to the Kasa app and finish. This does work for me in regards to being able to proceed...but, this does not fix the problem of the v5.26 not being able to connect when all of my v5.00 never had a problem.

If you go to your WiFi settings and look for available networks you should see something like this:

Also, when you connect to it...wait until you see a notification popup about how this connection has no internet. Click on that and select the 'stay on wifi' anyway choice. If you don't, you will find your phone changing back to a connected connection right in the middle of everything and then you need to repeat again. This is soooo convoluted of a way to add a device and not a good experience for customers to have to do.

Originally when you follow the steps and get to the final screen where it times out...if you look at your HS200 device at this point it is still flashing orange and green (and you get the message you got). IF you instead use these steps something indeed does change, but that change is instead just a flashing green LED now if it still has a connection problem (oh, and a different error message). The flashing green LED basically says the HS200 got the information for the network, but is still failing to actually connect. I'm going to assume the OP has v5.26 hardware since that is the one that seems to be the biggest problem with connecting to the network.

Once you get the flashing green LED the only way to fix is to do a reset and try again since no amount of time waiting appears to change the problem. I'm still working on getting this fixed. I did find if I use a 2nd phone to create a local hotspot I was actually able to add the device. But...that is not a permanent solution. In part I just wanted to test everything (i.e. what firmware it had, could it connect to anything, is it functional...). Good news is it connected, bad news is I still don't know why it isn't connecting to my network. And the kicker here is that I use a TP-Link AX3200 upstairs and a TP-Link AX3000 downstairs and it won't connect to either no matter what settings I've done so far. As I said...I'm still working on this. If you find that doing the above steps does get yours to connect, please be kind enough to let everyone know...but more importantly, what router are you using? If it doesn't work and you get the blinking green LED...still let us all know what router you are using since I'm curious if this problem is with the TP-Link AX series or ??

  @Wayne-TP 

For both routers, I can only choose between 802.11b/g/n mixed or 802.11b/g/n/ax mixed (although there is an ax only...many of my devices would fail at that point). For a new device, it should not have a problem with a b/g/n/ax mix vs. a b/g/n mix (especially when v5.00 and prior hardware had NO problem connecting to the 802.11b/g/n/ax). Otherwise this implies that from this point forward, to use the HS series you must downgrade a technology that's been around for years (making for a bad design decision). Regardless, changing between the two settings does absolutely nothing for me (as I would expect). Both settings result in failure to pair just the same...and back to the flashing green LED that I need to do a full reset to get back out of. Thanks for at least some suggestion. Unfortunately the reality is there is only so many settings you can do on a WiFi router. It doesn't take very long to try them all. I'm pretty sure I've tried all of them. I do have a lot of WiFi stuff going on, but nothing currently connected has any problems. Everything gets strong WiFi signals...and, when I did the 2nd phone hotspot trick it was in the same vicinity as everything else without having turned it off or anything. So traffic is not an issue either. And since I'm very picky about my entire network I do WiFi scans to see exactly what is happening in all the bands and have my network properly configured so each AP is using it's own name (no SSID sharing) and it's own channel (so not a single overlap of any of my WiFi connections). I also live in a house and not an apartment and therefore neighbors are not an issue for sure (unless I'm on the edge of my property).

  @Wayne-TP 

To be more specific about router settings...I have no advanced settings active. I have no MAC filters (or any filters other than a password), no SSID sharing. Every network uses a unique SSID and every network uses it's own channel (no channel sharing...not even channel overlaps). I use WPA2 Personal (WPA2 PSK) which defaults to AES encryption. I have however tried other types of security, but it had no effect. The Mode was just discussed and the Channel is one of the ones recommended in the setup process...but, I extensively use WiFi scanning to make sure none of my networks share the same channel (and living in a house away from neighbors means that isn't an issue either). No OFDMA, TWT, Smart Connect, Airtime Fairness...none of that stuff is active. It is about as basic of a configuration as I think I can get...

Oh...and forgot to add that network traffic isn't an issue (in part because everything is using it's own channel) since when I did the 2nd phone hotspot test that is successful, I'm doing it right in the middle of all of this without turning off my routers or anything. Also none of my connected devices have any connection issues. I don't get dropped connections or anything that could signify a problem. As a matter of fact...these are the ONLY devices that have ever had a problem connecting to my network. I have never, ever had any problems with any of my devices at any time...seriously (I'm involved in computers and electronnics for a living so I'm probably a little anal about this stuff as you can tell). I run ESPHome in Home Assistant with some 20+ devices. All of them show strong signals and never drops. Makes you seriously wonder why exactly these HS series (I'm having the same problem with current HS220s as well...basically any newer TP-Link has been a dismal failure which makes you wonder what testing they did exactly before making the decision to release these??).