If you are getting this error (Your order could not be processed. Please try again.), then it is most likely due to an issue with the Google Play Store or Google Play Services on your device.
Google has put together a list of solutions at the following link. There are two options (Computer & Android), so make sure you select the correct option. Also, most people seem to get it working by following the solution titled "Force the Play Store to stop, and clear the cache and data". If that doesn't work, then try the other solutions.
If you still have trouble after giving those a try, then please send a message via the app or click the link at the top of the page to open a new ticket.