@chirine
Your post does not specify which camera you have, but the two 2nd gen Google Nest cameras (battery camera and floodlight camera) should have a QR sticker on the front of the camera. If it is missing, you can select "install without scanning" and use the 6-character alphanumeric setup code on the bottom of the camera.
Here are install instructions for the two cameras:
https://support.google.com/googlenest/answer/10919314
https://support.google.com/googlenest/answer/11003780
