Using WoeUSB on Ubuntu 18.04 to make bootable Windows 10 USB stick.

Windows 10 installer ISO is freely available on Microsoft site. Microsoft recommends its media creation tool which does not work on Ubuntu. WoeUSB is a utility which can be used to create boot-able usb stick from Windows 10 ISO. To install on Ubuntu:

sudo add-apt-repository ppa:nilarimogard/webupd8
sudo apt update
sudo apt install woeusb

Insert USB stick larger than 8 GB formatted with single partition.
Run WoeUSB, select Windows 10 ISO.
Select NTFS File System since FAT does not support files larger than 4 GB.
Select target device USB stick (say /dev/sdb)
Unmount the partition opening a terminal

sudo umount /dev/sdb1

Click on install.

WoeUSB will make bootable USB stick with Windows 10 installer.