title: Xcode can build a project but can't run it on simulator
body: <p>When I download a project from the web, sometimes it can be built but doesn't run (XCode -> Product -> Run is disabled, in grey). This never happens when I create projects myself. What could be the problem? Is it somehow connected with version?
  I'm new to iOS development and download projects from the web for studying purposes. It's a pity not to be able to test them on the simulator.</p>