If you are trying to use the App Store, GameCenter, YouTube, or any other Apple app on your iPhone and you come across this error, follow the steps below to solve this problem:
Step 1) Uninstall the iBooks app and accept when it asks you if you want to remove your data.
Step 2) Reinstall iBooks by using iTunes on your computer.
Step 3) Open iBooks on your device and scroll to the bottom where you will tap "Sign Out."
Step 4) Close the app, and you should now be able to access the App Store on your phone.
Step 5) If the iBooks solution did not work, go to iTunes and sync and back up your phone.
Step 6) Restore your iPhone using this guide here.
Step 7) Set the phone up as a new device to be sure that the problem will go away, or restore from a backup if you are feeling confident about this solution.
Step 8) If the restore option did not work for you, try going to the Settings app and down to Store and clicking on "Sign Out."
Step 9) Open the App Store app and at the bottom, click "Sign In."
Step 10) Change your router's band to "compatible with N, b, and g mode." Sometimes single band routers have trouble supporting different devices on different bands.
Step 11) Lastly, try going to Settings > General > Restrictions.
Step 12) Enable restrictions. Wait 5 seconds. Disable restrictions.
Step 13) Apple recently made it so that passwords must include a capital letter and a number; if yours does not, I would recommend trying to change that using iTunes on your computer. Then follow the steps above to sign out and sign back in on your iPhone.
If none of these solutions worked for you, post in the comments below and I will keep searching for a solution. Otherwise, if any of these did work for you, post below so we can know which methods work best!