Here are the possible solutions:
- Make sure you are logged in the App Store with the same account you used to purchase the app.
- Make sure In-App purchases are not blocked in the Settings, see steps below:

How to enable or disable in-app purchases on Android
1. Open the Google Play Store app.
2. Tap on your profile icon on the top-right corner of the screen, then select Settings in the pop-up menu.
3. On the Settings page, tap Authentication, and then tap Require authentication for purchases.
4. Then select one of the three options: For all purchases through Google Play on this device, Every 30 minutes, or Never.
5. Enter your Google password when prompted.

How to enable or disable in-app purchases on iPhone or iPad
1. Open the Settings app.
2. Tap Screen Time, then Content & Privacy Restrictions.
3. Turn on Content & Privacy Restrictions by tapping the slider. It will turn green when on.
4. Tap iTunes & App Store Purchases.
5. Tap In-app Purchases, then select Allow or Don't Allow.

 

Troubleshooting: Amazon