Bootcamp Startup Disk Cannot Be Partitioned

I'm trying to free up some space on my iMac (27-inch, Late 2013) by removing my Bootcamp partition. I open the Boot Camp Assistant and it gives me the error "The startup disk cannot be partitioned or restored to a single partition."

I've seen guides that use the diskutil list and diskutil cs list commands but when I run diskutil cs list it gives me the error "No CoreStorage logical volume groups found."

Any help out there on how to proceed?