Parallels Desktop for Mac does not include Microsoft® Windows. If you don't have a Windows® installation disc or .iso disk image, you can purchase Windows on a DVD from many retail stores that sell software or online from retailers such as Amazon.com or Microsoft Online Store:
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 13 for Mac
Read this blog post to learn how to install Windows 10 easily using Parallels Desktop 13 for Mac: How to Get Windows 10 on Mac with Parallels Desktop