Unable to download Google Allo from Google Play Store - Pre-register/Unregister
We’re beginning to roll out Google Allo for Android and iOS, and the app will be available worldwide in the next few days.


(Taken from the end of the blog post)

Google roll out app releases in stages. Initially it will be available to a small percentage of devices and countries. Over a few days (or weeks even) it will roll out to more devices and countries.