I agree with bb21 that your ticket if, discounted against a Railcard which you can't produce (which in this case is because it doesn't exist!), is not valid for travel.
However, bb21 is also reliable in predicting that you are likely to have the ticket accepted with the full explanation you have given us here. But what I do advise is, that you carry some evidence to demonstrate that you made a genuine attempt to buy the appropriate ticket and that you were frustrated by the on-line booking system. A screenshot of the page which includes the drop-down box which did NOT include your discount should help.
The experience of many on here (which is consistent with my own experience) is that rail staff easily recognise the difference between passengers with invalid tickets who can only blurt out mumbled and irrelevant excuses, and those who have prepared themselves with all the documentation to support the tickets they're presenting which clearly illustrates one of the many limitations of the on-line booking systems.
Don't worry. Take the printouts/screenshots and ticket. Where the price is the same, as your is, I would expect all staff to accept them.