Allo has yet to be available. Google says it will be coming sometime to both Android and iOS.
The Launch
It was one of two messaging apps launched at I/O in 2016. Google also launched Duo, a video messaging app. That lets you see a video of the person calling before you answer the phone. Google says it gives you more context about a call before you pick it up. You can see if the person is smiling, for instance, or where they’re calling from. The Duo is also coming to Android and iOS.
Google ended the Allo app in March 2019 with a recommendation to users to change to Google Messages app.