We recently became aware of an issue that was preventing some users from being able to subscribe to Infuse Pro. This is an App Store bug which Apple is currently looking into, and with any luck will have resolved soon.
In the meantime, the workaround to get up and running is to activate Infuse Pro through Settings > App Store > Apple ID > Subscriptions. A bit more info on this can be found here. https://support.apple.com/en-us/HT202039
Thanks for your patience and understanding.