macOS 10.13: Windows 10 installation with Boot Camp
This question is possibly related to 
  Disk Could Not Be Partitioned - First Aid not helping — I'm duplicating my
  answer here.


I had very similar problem. After contacting Apple Support and following their instructions, I've reinstalled macos (instructions here). This is not a wipeout reinstall, but do make a backup. 

After the reinstall partitioning worked like a charm. 

And so do Bootcamp installation.