I was running my app this morning and now all of a sudden I am getting the following error when I try to run on the iPhone 5.1 simulator.
Cannot run on the selected
After a half day of experimenting, I think this means that the debugger can't find your executable to launch. I think this is an important distinction from the prior answers because its an underlying cause, that can have a lot of symptoms. In my case it couldn't reconcile the apps Info.plist for a custom build where we were copying the plist for the build. I changed the build setting to the Alt-Info.plist for AltDebug and that fixed it. Also meant no more copying.
Your underlying cause might be different, so the key takeaway is think about why Xcode might not be able to find your built app.