Welcome!
From the documentation…
To begin the PIN process, send the text “REQUEST” to the pincode channel while watching your AndroidTV. A 6 digit PIN should be displayed on the screen.
To complete the PIN process, send the PIN displayed to the pincode channel. The display should return back to where it was originally.
Was the 6-digit PIN displayed on the TV? If so, you need to send the PIN code to your GoogleTV_Pin_Code item to complete the process.
If you run into trouble getting the AndroidTV binding working with your TV, I’d suggest you post in the AndroidTV thread here.
As for the Alexa integration, sorry but I can’t really help with that part of the setup.
As an aside, the other way to handle the PIN request would be to use the Karaf console.
openhab:send GoogleTV_Pin_Code "REQUEST"
Then with the pin code (replacing 123456 with the PIN code displayed on the TV.
openhab:send GoogleTV_Pin_Code "123456"