If you are experiencing an Error 500 on an Android device when you try to download Appointuit following these steps should solve that problem.
1.Go to [Settings > Applications > Manage applications > Google app store]
2.Click [clear cache] and [clear data].
3.Open Google Play store to install the app again.
Other solutions may be found at:
https://www.androidpit.com/how-to-solve-google-play-store-errors
http://www.digitaltrends.com/mobile/galaxy-s5-problems/
http://www.makeuseof.com/answers/download-update-apps-samsung-galaxy/