- Make sure your device is connected to the internet.
- Restart your device and try again
- make sure you are logged in with an Apple ID that has a valid payment method setup
- make sure you don't have any restrictions setup by going to your device Settings and going to General > Restrictions. For more information see https://support.apple.com/en-us/HT204396
Google Play users:
- make sure you are logged in with a Google account that has a valid payment method setup