Boot Camp partition removal failed in macOS Sierra
After an unsuccessful Boot Camp Assistant “Remove Windows 10 or later version” execution, almost any CoreStorage partition I came across was FUBAR. This usually doesn't affect the file system/content of the system volume but some internal CoreStorage structures which inhibits any modification of the Logical Volume Group and subsequent containers.


Backup your main macOS volume with Time Machine or another backup solution.
Disconnect your external backup drive.


Last resort (very unlikely to work!):


Open Terminal and enter diskutil cs list
With the UUID of the Logical Volume (usually the last UUID in the output of the previous command) enter

diskutil cs resizeStack &lt;lvUUID&gt; &lt;size&gt;


e.g.

diskutil cs resizeStack F95E3156-8CFA-4B73-98F7-5A6D9644CA6F 248500m


The command usually fails with some error. But if you get Error: -69720: There is not enough free space... use a slightly smaller size like 248200m until you are successful.


Common approach (with a Time Machine backup):


Restart to Internet Recovery Mode by pressing alt cmd R at startup. Booting to Recovery Mode is not conducive because the Recovery HD will be moved in one of the steps below. And you can't move a partition used as a boot volume.  

The prerequisites are the latest firmware update installed, either E
thernet or WLAN (WPA/WPA2) and a router with DHCP activated.
On a 50 Mbps-line it takes about 4 min (presenting a small animated globe) to boot into a recovery netboot image which usually is loaded from an Apple/Akamai server.

I recommend Ethernet because it's more reliable. If you are restricted to Wi-Fi and the boot process fails, just restart your Mac with the same shortcut until you succeed booting.

Alternatively you may start from a bootable installer thumb drive (preferably Sierra) or a thumb drive containing a full system (Sierra).
Open Disk Utility in the macOS Utilities window
Repartition your internal drive to one volume/GUID partition table/JHFS+.
Quit Disk Utility.
Attach your backup drive.
Open Restore from Time Machine Backup in the macOS Utilities window and restore the backup to your newly created main volume.