If the Fax button is dimmed, it is most likely that in-app purchases were restricted on your device.
How to allow in-app purchases:
- Open the System Settings of your device > Screen Time.
- Tap Content & Privacy Restrictions and enable the toggle.
- Tap iTunes & App Store Purchases > In-app Purchases > Allow.
Note: You need to log in to the Apple ID which was used for the Scanner Pro purchase. This is essential for fax in-app purchases.