If you're receiving an “In-App Purchase Not Allowed" message when attempting to upgrade to Infuse Pro, you will need to enable in-app purchases through Apple's Screen Time settings on your device.
How to Fix
STEP 1. Navigate to Settings > Screen Time and select 'Content & Privacy Restrictions'.
STEP 2. Select 'iTunes & App Store Purchases'
STEP 3. Select 'In-app Purchases'
STEP 4. Choose 'Allow'
STEP 5. Success! In-app purchases are now enabled
If the problem persists, please contact Apple Support at https://support.apple.com/