@David_K appreciate the information.
@BGC11 how's it going? I know the feeling when you can't set up a device. No worries! Let's check what other options we have.
Try taking a photo of the entry key so that you can see it better on your phone. Also, if you still have the box, you may check the QR code there. Hit this link to know more about setting up a Nest Protect.
Keep me posted.