If you are unable to connect to iTunes here are some things to try:
  1. Make sure your iOS operating system is up to date (Settings > General > Software Update).
  2. Make sure your date, time and time zone are correct on your device (Settings > General > Date & Time).
  3. Check your device restrictions (Settings > General > Restrictions).
  4. Check and verify that you're in range of a WiFi router or base station. If you're on a device with cellular service, make sure that cellular data is turned on.
  5. Try resetting (turn off and on) your wireless router.
You could also try these steps:
  1. Sign out of your ID (Settings > iTunes & App Store).
  2. Reset your device's wireless settings (Settings > General > Reset > Reset Network Settings).
  3. Reconfigure wireless settings on your device.
  4. Without signing in, try and make a purchase (it should prompt you to sign in again at this point).
Alternatively, try this:
  1. Go to 'Settings'.
  2. Scroll down to 'Safari'.
  3. Scroll Down and tap on 'Clear Cookies and Data'.
  4. Tap on 'Clear Cookies and Data'.
If you are still unable to make a purchase or connect, please see this Apple Support article on declined payments. You might have to contact Apple as they may have blocked purchases.