you said selected it as a preview device - what do you mean here? It sounds like you selected your model of phone for the simulator. In the middle top of the Xcode window, it shows your target name a run destination. By default, that run destination for an iOS app is a simulator. Plug in your phone. If it doesn't appear in the popup menu as a run destination, choose Manage run destinations... from that menu. It should show up as discovered in the list on the left of the Run Destinations window. The first time you pair the phone with Xcode takes quite a while (several minutes for me).
Topic:
Code Signing
SubTopic:
Certificates, Identifiers & Profiles