I’m unable to buy the subscription. I am able to get to a dialog that indicated I will need to enter my password and answer my security questions. I enter the password, the App Store opens however I am never prompted to answer my security questions. When I return to shapr3d I get a dialog telling me the purchase has failed.
-Neil
Hey, unfortunately the ourchase flow is entirely controlled by Apple, and we can not do anything to fix that. Unfortunately you are not the first one who is having issues with the purchase 
others managed to solve this problem by contacting the Apple support.
Hi, is your iPad Pro relatively new (few days old)? If so than this might be a security thing by Apple where they block purchases if you restore the stuff from your old iPad to the new one. Please contact them and they will unblock you.
Let us know if this helped! Thanks