Bootcamp Uninstall Failed Partition Won't Delete

I have a problem with a mid-2011 iMac running High Sierra 10.13.6. I previously installed a windows 8 bootcamp and I was finally getting around to getting rid of it. I used the bootcamp assistant to try to remove the partition and merge the hard drive back together. However it failed. So I tried to go into Disk Utility to try to manually delete the partition. The Windows Boot partition doesn't show up under the hard drive device (not sure if it ever did). When I click partition button however it does show that there is 257GB of free space on my hard drive but won't let me delete/merge/format the partition. It gives me error message "An internal error has occurred and the disk for one of the specified operation could not be found. This may be a temporary issue, try again".

I tried going into recovery mode and erasing the entire hard drive but it gave me an error saying that the disk wouldn't unmount. I tried to use the terminal using "diskutil unmount force /dev/disk0" to force unmount it but it failed.

I have seen some of the forums on here saying to use "diskutil list" to generate a list. However they usually recommend deleting lines/disks that have to do with windows right? I checked and there is no disk part labeled windows or bootcamp. There are however a surprising amount of subdisks.

These forum posts also use commands that I'm not too comfortable with yet.

For Example:"diskutil eraseVolume jhfs+ BC1 disk0s3"

Can anyone teach me know how to use this erasevolume command to try to repair my hard drive?

A picture of the diskutil list is below.

Any help would be appreciated. Thank you.