FYI I'm using a MeCool KM9 Pro running Android 10.

I can access the remote through Google TV app and through the "TV Remote" quick toggle. But if I go through the Google Home app, I get this message.

Update the app and check the permissions on both devices

Update Google Home app (on your phone) & Android TV remote service (on the Mecool KM9 Pro)

Make sure HDMI-CEC is turned on (in both your TV & KM9).