How do I recover free space from my logical volume group?
CoreStorage is a Logical Volume Manager. It's based on Physical Volume(s) on the physical side and assigns Logical Volume(s) on the "logical side", which can be formatted with a file system.

Your Physical Volumes are disk0s2 and disk1s2 (which itself are slices of the physical disks disk0 and disk1).

The total size of your Logical Volume Group is 2120 GB. Only 1818 GB are assigned to a Logical Volume (disk2/Macintosh HD/JHFS+). So 302 GB of the Logical Volume Group are "free space". Some of this free space is reserved for management purposes. The net free space in the LVG is 296.6 GB or less.

You now can either expand the existing Logical Volume disk2 to fill the LVG or add a 2nd Logical Volume.

Check the disks first:

diskutil verifyVolume disk0s2


To resize a Logical Volume you have to execute the following command in Terminal.app:

diskutil cs resizeVolume lvUUID size



with lvUUID: the UUID of the Logical Volume (in your case C4D9F5AA-BA5F-4286-A630-FE8CAF4C913F)  
and size: a size in (t|g|m|k|b)  e.g. 2114g or magic sizes like R or 0 (which should resize the logical volume to the possible max).




In my opinion you already tried to resize the LV or LVG in the past and Boot Camp Assistant cannot cope with it or some error occurred while the BC Assistant was trying to shrink the LVG by 300 GB to free up space for the Boot Camp partition.

Usually this can't be fixed and you have to recreate the Fusion Drive from scratch and restore your whole system from a backup.