@HWTech: I believe Apple has an internalized list of machines that it'll try to run Boot Camp by software, namely those without an internal optical drive (which is pretty much everything 2012 and onward that should have Win10 support).
For everyone else in this thread, it certainly sounds like this is a bug/feature that Apple hasn't fixed (and most likely will not ever fix due to Intel Macs being obsolete). The program might be looking for a Win10 image of a pretty narrow size range, and if you've used a helper app to "slipstream" any fixes in your source media, that may well push it over the limits you can't see. You could try an earlier build like 1909 and see if that goes. Nevertheless, if Boot Camp successfully sliced the SSD into two partitions, you should be able to boot a USB-C stick with any build of Win10 on it and try a manual install from there, but remember that you must format the partition called BOOTCAMP first; this makes it NTFS, which Windows needs.
If the Mac has a T2 security chip, you may need to lower the boot protection settings in Recovery Mode to allow the non-macOS USB-C stick to start. Hit Command-R when the Mac starts and look for Startup Security Utility under Utilities. In SSU. select the middle option of the three, apply settings, and restart.
Back in macOS, make a separate USB-C stick with your drivers using the Assistant ("Download Windows Support Software" from the upper menu bar). and then plug in your installer. This time, restart the Mac holding Option (Alt) and select the non-EFI Windows volume, then click the arrow below it. Quickly press a key and Setup should launch. You'll eventually come to more options: Custom install, then select and format your Windows target volume. It should proceed unassisted from there.
If Setup worked, get through the Out-of-Box Experience (OOBE) as best you can. You want to be at the Desktop. Plug in your driver stick and use the keyboard to open Computer in File Explorer. We're looking for a file called BootCampSetup.exe -- run that and select OK or Yes as the screen goes dark. From there, navigate to any button marked Next, and Boot Camp should install the drivers your Mac needs. If the trackpad activates during this process, use it to finish and reboot your Mac.
If you're back in Windows, run Apple Software Update to get all the drivers you need (there's a selection of video drivers and a few Wi-Fi updates to be had) and reboot as many times as you must until ASU has nothing left.
Good luck.
Nate
