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.
Generally, if you can't Run a project, either:
It doesn't have a build scheme that knows how to "run". Check at the top next to the Run/Stop buttons that there's a profile name and target device shown. If the box says No Scheme then click on it and create a New Scheme. If there IS a scheme, click on the drop-down menu (left divider in Xcode 4.2+, anywhere on the box otherwise) and go to Edit Scheme to make sure the scheme was set up correctly for Running
It doesn't have a build configuration set. You should see this in the Run section of your build scheme. Check there's one selected. If the list is empty, go to the Project icon at the top of the leftmost pane, select the Project, go into the Info section, and create a build configuration, although there should already be at least one if you can build the project
It doesn't have a runnable target at all, which might mean you've got a project which compiles to a static library or framework for use in OTHER projects. If this is the case, you should see the icon for your build Target as a yellow toolbox (framework), or a little white building (a "library", although to me it looks more like the United States Supreme Court Building).
The target isn't valid for the Simulator, which means you might need to add i386 to the list of Architectures and Valid Architectures on the Target before you can run it
The target isn't valid for the currently connected device. For this one, check the Deployment Target number is set back enough to be valid for the device's version of iOS
Xcode can't see the connected device. Try unplugging and plugging it back in. If it still doesn't recognize it, you may have not added it to Xcode (Window -> Devices -> Select the device -> Use for Development)
