Boot Camp says I don't have enough space to create a partition when I do

You're absolutely right! I was going to just reply that "tmutil listbackups" shows no snapshots and I'm sure my file system is fine. Well, I booted into Recovery Mode, ran Disk Utility's First Aid on my file system, and then tried Boot Camp Assistant. It worked!

I must have read 25+ various web pages on help with installing Boot Camp. Not one of them mentioned or even hinted at making sure your file system is completely clean. Thank you!

The error arises because there is not 50GBs of contiguous space. Frankly, if you were left with only 5GBs of free space that would not be sufficient for macOS. You should rethink your space allocation if you plan to install macOS and Windows. macOS requires a minimum of 20GBs or 10% of the drive's capacity, whichever is more.

If they're having the same problem I am, the error is coming from Bootcamp Assistant while trying to repartition the disk, long before installing any MS Windows images.

If you are on High Sierra/Mojave, you have APFS corruption or TM Snapshots.

Glad to see it worked out.

If you are using W10 1809 (October) ISO, try the W10 1803 (April) ISO instead.

If they're having the same problem I am, the error is coming from Bootcamp Assistant while trying to repartition the disk, long before installing any MS Windows images.