You may have restrictions on for in-app purchases.
1. Launch Settings
2. Tap General, and then tap Restrictions
3. Tap the option to Enable Restrictions. Enter and then re-enter a Restrictions passcode
4. By default, all of the apps and services are allowed. To disallow in-app purchases, tap on its button
1. Open the Google Play App
2. Press your phone's Menu button and go to Settings
3. Scroll to the "User Controls" section.
4. Tap on "Set or Change PIN option" and enter a 4 digit PIN
5. Back to the "User Controls", simply check "Use PIN for purchases"