Can't install Windows on Boot Camp (Retina MacBook Pro, mid-2015)
Below are the instructions for installing Windows 10. 


  Note: To get a better view of the images shown below, either click on an image or open an image in a new window.



Remove all Windows related partitions that may have been created by previous attempts to install Windows. When finished, the output from the command diskutil list disk0 should appear similar to what is shown below. I assume your sizes will be slightly different.

/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                 Apple_APFS Container disk1         999.8 GB   disk0s2

The command below will create the 122 GB "BOOTCAMP" partition on your internal drive. The space occupied by this partition will be used for installing Windows. 


  Note: Since I do not know the exact size of your Apple_APFS partition, I assumed the partition was 999.8 GB in size. Since this is unlikely, you may have to adjust the value of 877.8 GB.


sudo  diskutil  apfs  resizeContainer  disk0s2  877.8G  FAT32  BOOTCAMP  122G

Use the Disk Utility application to erase a 16 GB or larger USB flash drive. Enter the settings shown below in the popup window. When finished, do not remove the flash drive.


Use the Boot Camp Assistant to download the Window Support Software. Look for the "Action" pulldown on the Boot Camp Assistant menu bar. On my Mac, these files were downloaded to the ~/WindowsSupport directory.


  Note: You can not download the Boot Camp Support Software for your Mac computer from any Apple website. You must use the Boot Camp Assistant to download an officially copy of this software.

Mount the Windows iso file and copy the contents to the "WINSTALL" volume. You will need the use the cp command from a Terminal application window. Below is the exact command I usually use. The string ESD-ISO is the name of the mounted ISO file from Microsoft. If necessary, make the appropriate modifications. 

cp  -Rv  /volumes/ESD-ISO/  /volumes/WINSTALL



  Note: This command will take a while to complete. Be patient!

Copy the Windows Support Software to the "WINSTALL" volume. Below is the exact command I usually use. If necessary, make the appropriate modifications.

cp  -Rv  ~/WindowsSupport/  /volumes/WINSTALL

Continue starting at step 3 of my answer to the question Boot Camp Assistance is stuck on create a partition?