Fire Emblem Heroes is currently in the process of being rolled out worldwide. With many users already being able to download the app on Google Play and certain regions for iOS. Much like the launch of Super Mario Run, expect this launch to be slow at first, as servers are adjusting to traffic. If you don’t see Super Mario Run on your Google Play or App Store, give it a while as the game my not be available in your region just yet.
North America should expect the game to arrive sometime past midday PST for iOS. Remaining regions, should expect the game very, very soon. Be patient!