The plugs need to be set up with Kasa, and remote control needs to be enabled, here is a link that shows that process: https://www.tp-link.com/us/faq-946.html

Next, if you want to set up Google Assistant that is on your mobile phone, reference this article: https://www.tp-link.com/us/faq-1765.html

If you want to set up Google Assistant with your Google Hub device, reference this article: https://www.tp-link.com/us/faq-1534.html

When you authenitcate the Kasa service in Google Assistant, you need to use the same email and password that you used in your Kasa app.

i'm having the same problem...there are a several others reporting the same problem in Google Home forums.  The Google app just won't keep a permanent connection to the TP link account.  Alexa did it in a snap.  

@MarkJonesey 

I haven't personally had any issues with my Camera Switch or Lightbulb having connection issues with Google Assistant and I have 3 google home devices in my home.  I will reach out to our development team to see if they have heard of any issues between us and Google lately and will update this post as i have more informaiton