First, try these steps below.
-Try turning off your device by holding the lock button down and powering down when prompted to “Slide to turn off”. Then attempt the download again.
-Try rebooting the device by holding both the lock and home buttons at the same time (continue to hold through the slide to turn off prompt) until you see the Apple logo, then turn the device back on.
-Delete the application and try starting the download again.
If none of these steps are working, please contact Apple’s support team: https://support.apple.com/