This certainly isn't your fault, and I'm sorry you're taking the brunt of this...
Could Upwork be easier for clients to use? Yeah, sure. I hope it continues to get easier and more intuitive.
But posting a job on a job site and having people apply to your job is such a basic concept... In this case I'm more dismayed by your client than by any failing on Upwork's part.
Suzanne is right: Ideally Upwork Customer Support would assist this client. But I'm not sure I trust that the client really put any effort at all into contacting customer support. I think there is a certain amount of laziness associated with a client who doesn't figure out how to close a job posting.