How can I create a Windows 10 boot usb from ubuntu?
One tool that you can use is Unetbootin, which I have found to be quite reliable when creating bootable USB sticks.
You can install Unetbootin with sudo apt-get install unetbootin, and use it via sudo unetbootin.
NOTE: Unetbootin is a graphical applications, it just requires root-level privileges to be able to modify the USB and make it bootable, hence the launch via Terminal and not the Dash.