At this point, it is not going to hurt you or cost you to ask the client to pay you. She will either pay you, or not pay you.
She is probably just a scammer. But in case she is not, if you think she might simply be ignorant of how to use Upwork properly, you could tell her:
I just realized I started without accepting an actual hire offer from you. That's my fault, because I'm new. If you can go ahead and use the Hire button on your client-side Upwork interface to send me an hourly contract offer with manual time allowed, then I'll be able to log the time I've worked thus far, and continue working on the project."
If she doesn't send you a contract offer, then feel free to stop communicating with her entirely. The work you have done for her thus far belongs to you, and you can use it as a portfolio piece.