re: "Client not paying"
This means that you used Upwork incorrectly.
This is good news. Because it means that you have the power. You have the control. All you need to do is learn what your mistakes were, and not make those mistakes again in the future. Then you can avoid not getting paid like that.
re: "We didn’t start a contract but the proposal is still active"
I'm sure you will never make that mistake again in the future.
Now you know that you need to accept the contract in order to get paid.
re: "She said she would pay me but 3 weeks have gone by and I still haven’t received my pay for the 15h worked."
I have a phrase I use for clients who owe me money for work I did on Upwork more than 3 weeks ago:
I call them: "people who don't owe me money"
Because on Upwork, I always get paid within 3 weeks of doing any work. If I did any work three weeks ago and I still haven't been paid... then it means that I did something wrong.
re: "She keeps telling me via WhatsApp..."
It is a serious violation of Upwork ToS for a freelancer to communicate with a prospective client outside of the Upwork platform if an Upwork contract has not yet been established.
Because you communicated with this client, and no Upwork contract is in place, you have violated Upwork ToS. Your account could be suspended or otherwise penalized for doing that.
re: "I am at a point I don’t know what to do."
What you should do is learn from your mistakes, and don't make those same mistakes again in the future. Let go of any desire to be paid for the work associated with this project. It is a serious violation to exchange funds outside of the Upwork platform. There is no way for you to get paid unless the client sets up a contract. I suppose that you could ask the client to create an official contract and pay you, but it doesn't seem that she wants to. I think she was tricking you into working for free.
However, you MAY indeed ask her to send you an official Hire offer so that a contract can be made, so that she can pay you. The worst that she can do is say no. If she says yes and pays you through the Upwork plastform, that's great. If she doesn't, then you can report her by clicking on the "Flag as inappropriate" option on the job posting page, or by reporting communications within the Upwork Messages tool in which she told you to communicate with her via WhatsApp.
re: "Could I at least write a bad review about her?"
You just did.
Your initial post in this thread constitutes your report about your experience and your bad review of her.
