Q: Disk with only 1 partition cannot be partitioned or restored to a single partition

I know several people have posted questions on this topic over the years, but I believe I've gone through all their resolutions and none of them are working for me!

I am on an iMac I just reformatted and reinstalled a fresh copy of El Capitan onto. I attempted to run bootcamp but got the following message:

Normally this occurs when people have an extra partition. I only had the recovery drive, but to be safe I went ahead and removed that too, leaving me with a single partition:

Apple's official support document recommends deleting extra partitions, but it says nothing about what to do if there are no extra partitions. I did attempt to run first aid on the disk etc and there were no problems found.

Help! What can I do? I've been trying to avoid backing up the whole install (which I just completed yesterday) and wiping the drive and installing fresh.