Here are some troubleshooting suggestions from Apple that should fix the issue:
Tap iOS Settings > General > Date & Time.
Make sure your date, time, and time zone are set correctly.
Tap iOS Settings > iTunes & App Store.
Try signing out of iTunes, and signing back in again.
Make sure that your iOS device is connected to Wi-Fi. Learn what to do if it won’t connect.
If you’re using cellular service to connect to the Internet, make sure that you have cellular data for the iTunes Store turned on. To do so, tap Settings > Cellular and make sure that Garden Answers is set to On.