Error running playground. Unable to find suitable target device
Thanks a lot for Raptor's kind response. I have solved this problem. I found out that I just added not enough simulators so that Xcode was unable to find suitable target device. After I added all the 10 simulators for iOS 8.1, the error is gone and the output appears.
At first, it seems that I will have to add all of them to make it work. Then after quite a few experiments by removing and adding some back and forth and restarting xcode, I cannot make sure exactly how many simulators are needed. Maybe 3 or 4.
playgrounds will run fine, assuming at least one of the following simulators are configured: iPhone 6, iPhone 6+, iPad Air, resizable iPhone, or resizable iPad.
Delete the contents of ~/Library/Developer/XCPGDevices (typically it contains a directory with a name which is the UDID of the simulated device)