Make sure your device is compatible with Covet Fashion by checking here. Also, check that your device has got enough storage space leftover (the Covet Fashion download is pretty hefty at around 1.5GB). If that still hasn't worked, try restarting your device and closing other apps running at the same time. Your store should be able to help you find a solution if nothing else works; it could be due to an app compatibility issue on your specific device or network issues within your local store.