Some users are currently reporting that while making purchases via the app there are issues when it comes time for their bank to authorise the payment, usually via pop up or use of their banking app.
We are investigating ways to accommodate the various methods of user identification used, however in the interim you can make purchases at: https://shop.2000ad.com/ via your devices web browser and when complete just reopen the app and tap 'restore purchases' in your account area.