Thanks for posting ― let's see what's going on.
To confirm, have you already added the Nest Thermostat in the Alexa mobile app. Also make sure that you're migrated to a Google Account. In the Nest app, try to sign in using the Sign in with Google button. If the you haven’t migrated yet, the app will prompt you to migrate.
Thanks for reaching out here in the Google Nest Community.
I'd like to check if you've already tried migrating your account to Google. As what Mel has mentioned, it is necessary to have your Alexa recognized your devices in the network.
Thanks for your help here, Mel.
I'm glad to hear you got everything working! In that case, I'll go ahead and lock this thread. If you have any more questions or concerns in the future, please feel free to create a new thread.
Have a good one!