1) Clear Cache in your phone (if possible, for all apps)
2) Uninstall SunNxt, do a Clear DATA for Google Play Store, restart your phone, launch playstore to install SunNxt again
If it still doesn't work,
Uninstall SUNNXT, do a CLEAR DATA for Google Play Services, restart your phone, and try installing it again.
Note: Clearing DATA of Google Play Services will not delete any of your Personal Information. But if any other app misbehaves after clearing data and restarting your phone, do a clear data of that app alone and login to that app again. It should work fine again.