Thanks for reaching out and sorry for the delayed response.
What type of Nest thermostat is this? Do you see a message saying, “Your thermostat couldn’t be added to your account” on your thermostat or “Added to your account” on the thermostat but “! INCOMPLETE” on the Nest app home screen? If you're getting either of the two, please try the suggested troubleshooting guide here: https://bit.ly/3unN2A9
Keep me posted.