Hi Tracey,
This must be so frustrating. I believe there may be some issues with adding cards using TfL Go. Have you tried logging into the TfL website and adding the card there? I’ve certainly never had any problems adding a new card that way. It is correct that you need to have made a journey to be able to add the card to an account.
What I would suggest straight away is that you note down anything that was said when you had the railcard added. What was the password you had to set up? Was it the answer to a memorable question? Obviously don’t type those details on here, but you might need to remember them down the line.