How do I fix "Your Disk Could Not Be Partitioned" isssue with BootCamp Assistant?
The following post (see hyperlink) was useful, especially the bit about how you have to create a New partition when you're in the Windows installer.  My previous answer was downvoted and I deleted it, but this actually solved the issue for me.  https://fgimian.github.io/blog/2016/03/12/installing-windows-10-on-a-mac-without-bootcamp/