@Watchowner 

Focus on the Kasa app to see if the plugs fully function that way. See if you can control them from the app if you are on the same Wi-Fi, and if you are only using your cellular connection (remote). 

Both Alexa and Google rely on the remote aspect working for Kasa.

When you relink the TP-Link with Google, make sure the email account you are using is the same as what is in Kasa (just for the TP-Link part, not your main google email).

@Watchowner I'm having the same issue, how was yours solved?

Initial Steps I followed and issues I encountered:

1. Unboxed the TP-Link smart plug.
2. Followed setup instructions included with the TP-Link smart plug packaging.
   1. This included downloading the Kasa application to my phone.
3. The TP-Link smart plug was not recognized by the Kasa app.
   1. I followed the setup instructions included with the TP-Link smart plug packaging multiple times.
   2. I followed identical instructions provided by Kasa online support multiple times.
   3. I verified all connected devices were on the same 2.4 Wi-Fi network.
4. End result, the TP-Link smart plug was not recognized by and would not connect to the Kasa application.

Next Steps I followed and issues I encountered:

1. I linked the Kasa app within the Google Home app and ensured all devices were on the same 2.4 Wi-Fi network as requested.
   1. The Google Home application would not recognize the TP-Link smart plug
   2. The Kasa application would not recognize the TP-Link smart plug.
2. I deleted the Kasa app and factory defaulted the Google Assistant.
3. I set the Google Assistant back up and reinstalled the Kasa app. 
   1. Google Home would not recognize the TP-Link smart plug.
   2. Kasa recognized the TP-Link smart plug.
4. I continued to add the all devices that had been linked to my Google Home previously (lights, speakers, etc).  All devices were set back up and communicated as expected with the exception of the TP-link smart plug.
5. One last effort was made to repeat all steps, the TP-Link smart plug would still not communicate within the Google Home app though it would with the Kasa app.

My choice is to use one appication to control all devices within my home.

I’m not sold on Kasa products, I will be returning my 1^st Kasa product and will seek a better alternative going forward.

@Watchowner 

I bought a bunch of reconditioned Kasa mini plugs on Amazon.  They work very well although I had this problem initially.  It is simple to set up once you have found the right settings.

Set up your TP-Link device on the Kasa app with your cell phone, setting the WIFI to the name of the Kasa gizmo when prompted.  Then it will find your WiFi network and ask you to log in.  The app will ask you to create an account on TP-Link Cloud, do so.  You will then select a Wiki for your device on the Kasa app.  Check that you can control the device with the Kasa app. The default control setting is Local (with Kasa app only).  To use your gizmo with Google Home through TP-Link cloud, you need to do two things:

    1.  Set the Kasa control setting to Remote.  Click on the Wiki in the Kasa app; on the next screen, click on the gear on the top right to go into setup and toggle the control to Remote.

    2.  Link your TP-Link Cloud account to Google Home.  You can ask your google assistant how to do that and follow the simple instructions.  It links seamlessly.

Then you can control your TP-Link Kasa gizmos with Google Assistant.

@Watchowner 

I had to reset my already connected TP-LINK Kasa and Google Assistant because of a network change. Everything was going well, until I tried to add my Kasa devices on Google Assistant - it kept telling me my account was already linked, but it had no devices linked. I check my Kasa app and I had 5 devices on there. 

I tried all of the above mentioned methods to no avail. 

The thing that finally worked: Sign out of Kasa account in Kasa app. Sign back in.

I have no devices on the app anymore. So one by one I add them back. 

And the Google Assistant now recognizes the devices. 

@Watchowner Fixed.

My 3 Kasa plugs were working with Google home mini speaker thing then suddenly two stopped working with voice commands with the audible error "Sorry, cannot access TP-Link Kasa".

If they are working in your Kasa app, you do not need to remove them from your Kasa app.  This is all Google's fault:

Find the Google Home app on your phone (I was on Android).

Remove the devices from Google Home app (I was on Android).

We realised this was the problem when it said "only the owner can remove them" for the 2 faulty plugs, aha!  plugs were added from a different google account.  I had added two of the plugs but my wife owns the google home (figures).

Google Home app -> Add devices (set up device -> works with Google) to Google home app using the Google account that your Google assistant smart speaker is using.

You can then add "TP-Link Kasa" to Google Home app and Google will ask for your Kasa account, thus giving Google access to Kasa!

Each device can then be added to your Google "home" which will make them work with voice commands on your phone and your patronising little Google snob of a speaker.