CoreStorage disk won't unmount
Something may be mounting the disk on your startup system. Try to exclude the disk from spotlight, time machine and so on.

After that, there are a few unmount options in the terminal - sometimes, you have to unmount the volumes before you unmount the disks, or vice versa, and such.

Also, a few times in my past, the disk utility from the gui could unmount something which the diskutil command could not.

As a final recourse, try all this with the online recovery (it tends to be less likely to mount something) and/or the original version of MacOS your Mac shipped with (it may not understand new features of your Fusion Drive's file system, which is an advantage when deleting it).

On a sidenote, the zeroDisk option in Diskutil cleans up your disk more radically (and wears down the SSD more), but may help when issues remain. But it also only works when unmounting worked. You use it on both disks and let Disk Utility repair the Fusion Drive again. Use it only when deleting is unsuccessful. Be aware that a small error can delete the wrong disk, that there is no going back from it, that it takes a long time (though usually you can stop it with ctrl-c after the first percentage above 0 appears), and that it may destroy a disk (lots of zeros will be written).