Boot Camp (v6): "Can't install software" prompt; Late 2015 iMac with 27" 5K, OS X El Capitan

I want to install Windows on my Mac. I am trying to use Boot Camp to run Windows 10 natively. I will not be using a virtual machine (No Virtual Box, no VMWare, no Parallels).

Model: Late 2015 iMac with 27" 5K Retina screen

Operatin System: OS X El Capitan

Boot Camp Assistant Version: 6.0.1 (522)

Storage: 3TB Fusion

The following are links to problems I am NOT describing. Please review them before answering.

Impossible install windows 10 in imac 5k in boot camp assist.

Boot camp 6 error message 'can't download a software because of network problem'

can't download the software because of a network problem

Below is a description of the problem I am experiencing, with screen shots of each step in the process.

These are the instructions for using Boot Camp: How to install Windows using Boot Camp - Apple Support The problem I am experiencing is between steps 3 and 4.

Boot Camp Assistant says it can't download the support software. This is shown in the last screen shot in the following set. The following screen shots illustrate the steps I go through and what I see on my screen.

Figure 1. Open Boot Camp Assistant

Figure 2. Select partition size and .iso image (does not prompt any other screen prior this, or any other options whatsoever)

Figure 3. I do that.

Figure 4. Download appears to begin. I don't know where this is downloading to. It doesn't make a difference whether or not I have a USB drive plugged in.

Figure 5. No indication of download success/failure, network/connection status, just "Can't install the software."

Figure 6. This is from my screen share with Apple customer support. Note that the partitions are intact. Boot Camp did not partition my hard drive.

Figure 7. I tired downloading it separately, before running the installer, but Boot Camp attempts to download the software again anyway.

Figure 8. Apple Support sent me a link to download the support software. Note that the support software is v5.1.5621 and Boot Camp is v6.0.1. I have copied the files within the BootCamp5.1.5621 folder to a USB drive and restarted Boot Camp (see Figure 1) with the same results. Again, no option other than that shown in Figure 2 was ever available.

Other Symptoms:

This problem happens with or without the USB plugged in.

If the USB is plugged in, the problem happens whether or not it is formatted as FAT.

If the USB is plugged in, the problem happens whether or not the USB partition table is MBR, protected MBR on GPT, or GPT.

These have all been verified.

To be clear, the hard drive is not partitioned, Windows is not installed, and I am not experiencing driver issues because I cannot experience driver issues - there is no Windows partition or OS to cause driver issues.