change target framework.

If you found that you cannot select the device for debugging, maybe your target framework is higher than your existing debugging device. You can confirm this when you get this error:
"Running this project required an Android device with API 21 or above".

Because I only install the recommended device and I am still new, I finally manage to found out that all we need to do is go to the project option ( by clicking the "Configure Target API..." or go to "Project" + " 'projectName' Options" (it is under "solution Options") ) Change the Android Application's Minimum Android version and Target Android version to the lowest option, which is might be "Override - Android 1.6 (API level 4).

Go to workspace, select the device, and cheers !Happy coding.