Cannot partition internal drive to install bootcamp because of APFS/High Sierra When I run Boot Camp Assistant, the attempted installation of Windows 10 fails, giving me the error "An error occurred while partitioning the disk." Have already tried First Aid on both my macOS boot volume and in recovery mode. Also tried /sbin/fsck -fy in single user mode. Both didn't work. 

Here's the catch - when I make my own partition on my internal disk via Disk Utility (recovery mode and boot volume Disk Utility both have the same result) the partition will fail if I set the partition for over 15 GB. When I make a partition (not APFS container) that is 15 GB or less, Disk Utility will make it without problems. When I attempt to make a partition over 15 GB, the process will fail.

Now, A few days ago, I restored a Time Machine backup of my entire main internal drive via recovery mode and turns out that after that restore Macintosh HD reverted back to Mac OS Extended (Journaled) instead of APFS. After said restore and my internal disk being HFS, Boot Camp Assistant was fully successful in installing Windows 10. So I know that (hopefully) that process will work again if I want to install Boot Camp again. Since then, I deleted the BOOTCAMP partition and deleted Windows from my MacBook Pro (why I did that is a complicated story). I now want to re-install Windows 10.

However, I have an additional Time Machine backup of a clean High Sierra install which I restored to an additional APFS container on my internal disk just before writing this. When restored, this backup did not revert back to HFS after the restore, which in turn means bootcamp assistant/disk partitioning for 15 GB+ failed. I'm willing to restore the backup of my entire main drive from before all these shenanigans started in an attempt to get APFS to accept a partition, but I would like to know more options.

The main point - What are my other options to force APFS/High Sierra to accept a partition of greater than 15 GB and/or make Boot Camp Assistant actually go through with it's job. I’m open to all options to solve this problem. High Sierra has a lot of bugs in it and they're getting on my nerves. Apple needs to up their QC.

Anyways thanks to everyone for reading/helping me out - much appreciated :)
If your disk has been converted/formatted to use APFS, and you use Time Machine partitioning will typically fail because Time Machine made local APFS snapshots.

You can see the snapshots created with the command (use Terminal to enter it):

tmutil listlocalsnapshots /


If you see some, run the following command:

tmutil thinlocalsnapshots / 9999999999999


This last command will delete all the local snapshots.

Now run the Bootcamp assistant again, this time partitioning the disk should succeed.