I also do have an GDR2/Amber aboard and can't update...
Here's an explanation why:
When Nokia released Smart Cam it has been available to every Lumia (accidentally or not), so many of users downloaded it...
When they realized it's not possible to open & use it on non-amber devices, they kept sending requests and bad reviews about Smart Cam app.
So Nokia disabled its availability to older models in day or two after publishing.
It will be unlocked to download back again, when GDR2/Amber rolls out globally to WP8 Lumias.
Capisce?