Windows will not install on Bootcamp partition

I've managed to get High Sierra on back on my mid-2014 rMBP and am trying to install Windows 8.1 via Bootcamp Assistant. I have a Samsung NVMe SSD in my laptop, and I verified that W8 would be compatible with it. The computer partitions the drive just fine and boots me into the Windows setup. When I go to select my Bootcamp drive, it says that it needs to be formatted to NTFS. When I do that and click Next, I get the error message: "We couldn't create a new partition or locate an existing one."

When I completely clear/delete the Bootcamp partition and make it into Unallocated Space, I'm able to get to the next prompt, but Windows will not install - I just get another error message and booted back into macOS. The same also happens whenever I try to do a manual installation.

I've been at this for days. Is there any way to bypass these error messages? Are there any additional drivers that I need to install onto my bootable Windows USB stick in order to get this to work?