I have a nest guard with 3 key tags. 2 work fine but one was connected to my previous roommates google account. She deleted her google home app when she moved out. I can no longer see the tag on my account (owner) and when I try to set it up, I get error NT009(3). I have tried. Restarted my nest with no luck.
Thanks for reaching out. I feel you on this. Quick question: could you ask your friend to remove the Tag off of her Nest app so you can add it to your account then let me know how it goes?