You may have restrictions on for in-app purchases. To remove restriction follow the bellow steps:
FOR iOS
- Go to device Settings
- Tap Screen Time
- Tap Content & Privacy Restrictions
- Turn on Content & Privacy Restrictions
- Tap iTunes & App Store Purchases
- Tap In-app Purchases
- Check Allow
FOR ANDROID
- Open up Play Store and then hit the menu button located on the top left-hand corner
- Scroll down a bit and select the Setting tab, where you will find the ‘Require authentication for purchases‘ option
- Tap that and then select ‘For all purchases through Google Play on this device
- If your phone sports a fingerprint sensor, then fingerprint authentication will be available as well
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