I had the same question and asked Customer Support. I just received reply. The Technical team advise us not to download Windows 10. They stated that the operating system is still on a testing period and not compatible with the current Team App version.
Thanks for the heads up. I will reach out to the team to validate the information you received so that we can update the downloads page with this information.
I don't understand why this was not updated to work with Windows 10 since there are a number of people who will inadvertantly download Windows 10 and there will be a major issue with the tracker. I would think the company knew for some time that Windows 10 was coming out and would have made sure that the tracker worked with it.
I was just informed and can confirm that the new Upwork Team App is compatible with Win 10. Currently, you may receive a warning message that the publisher is not verified, and will have to click "trust". But the app and download will work on Win 10.