re: "One of my client is harassing me, he is forcing me to start work without start contract."
There is a misunderstanding on your part.
It is impossible for a client to force you to start work without a contract.
Luis: I don't doubt that you feel some distress right now.
But the MAIN problem in this situation is NOT the client.
The main problem here is the freelancer.
You (as the freelancer) need to understand Upwork better.
You should know that a client can't force you to work without a contract.
You should also know how to block a client. If a client is harassing you, then it is easy to click on their name in the Messages tool and select an option from the contextual dropdown menu to BLOCK them.
Also, you should know how to click on the "Flag as inappropriate" link button in the top right hand corner of a job posting page, or under the "Three Dots" icon, so that you can REPORT a client who is acting inappropriately... including (but not limited to) asking you to work for free.
If you have questions about any of this, you need to ask those questions so we can help you more.