You can only play Duos/Squads with Battle Buddies who are on the same server as you. If you and your Battle Buddy are both online but you can’t see each other, it’s likely that you’re playing on different servers. 
Head to the Settings menu and tap ‘Region’ to ensure you’re playing on the same server – and presto!As an example, you can check what the server Regions look like. You may see a different recommendation for the best Region, depending on your location.
Please note that different versions of the game are not compatible with each other so make sure you’re both playing the newest version.