I had the same problem as a, now locked, thread with the same title.  My solution wasn't listed and some of the ones in that thread didn't work or seemed way more cumbersome than what worked for me.  So, I added this new thread with my solution:

My Kasa app would control my outdoor smart switches (KP405), but Google Home app showed them offline and wouldn't control them.

To fix it, I went into the Google Home app, clicked Settings, clicked on one of my Kasa devices, and then clicked on "Unlink TP-Link Kasa" towards the bottom of the device settings page. In the pop-up, I clicked "Unlink". It then took me to a "Home control" page and, under Linked Services, I clicked "TP-Link Kasa". Last, in the pop-up, I clicked "Reconnect account". I was then able to control the switches from the Google Home app again.