reinstall windows on a machine that has ubuntu only

@nawalsubba  If you have access to a Windows machine, download the Microsoft Media Creation Tool to create Windows 10 installation media directly from Microsoft instead.  It will come with the current release of Windows 10 and won't have any other Dell utilities -- though you can reinstall any that you might actually want.  When you get to the step in Windows Setup asking where you want to install Windows and you have a list of disks and their partitions shown, delete any existing partitions on the target disk until that entire disk shows as a single entry called "Unallocated space".  Then choose that as where you want to install.  Windows Setup will create the appropriate partition layout.

And once you're running Windows, you can turn UEFI Secure Boot back on if you had to disable it to run your Linux distro -- assuming this is a UEFI-capable system, of course.  You didn't specify what system model you're using.