Parallels Desktop for Mac does not include Microsoft® Windows.
You can purchase Windows 10 using Parallels Desktop interface. Visit the article below for more information.
Or you can purchase it directly from Microsoft official website:
A list of all compatible Windows versions is available on this page:
Detailed instructions on how to install Windows into Parallels Desktop can be found in KB 4729.
How to get Windows 10 in Parallels Desktop for Mac
Read this blog post to learn how to install Windows 10 easily using Parallels Desktop for Mac: How to Get Windows 10 on Mac with Parallels Desktop