You can play Duos with Battle Buddies who are playing 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’ 
Check to see if you and your Battle Buddy are both on the same server region and you should be good to go!Also, please note that different versions of the game are not compatible with each other. Make sure you’re both playing the newest version!