@jjtucker12
Unfortunately, as with Google Nest cameras and doorbells, you cannot install a Nest Protect with the serial number. You need either the QR code or the 6 digit "entry key" (which Google Nest recommends saving) on the back of the Nest Protect in order to install it.
https://support.google.com/googlenest/answer/9247146?hl=en-GB&sjid=16836106907813422476-NC#zippy=%2C...
Many other customers have been caught by this constraint. Because of this, we've been writing down and saving the 6-character alphanumeric setup code for our Google Nest cameras and doorbells for years.
