Trouble Undoing Bootcamp

A Neurotic

I used BootCamp assistant a few months ago to set up a partition on my MacBook Pro to install Windows. I used it for about a day and haven't booted it into Windows since. Storage space is filling up on my hard drive so I used BootCamp assistant again a few days ago to delete the partition to try and restore the 50GB I wasn't using. I'm not sure exactly what happened but now it appears to have done a half-job on it. I can't boot into Windows and can't see the Windows partition in Disk Utility, but the 50GB remains inaccessible. When I go into partition settings I can see the 50GB as "free space":





but when I try to delete it/merge it with the rest of the hard drive -





I get this error/operation failed message:




As you may be able to tell from my explanation of things, I haven't a f*cking clue what I'm doing so any help or advice in unlocking that storage space would be appreciated!

jc77

Erase required

1. Back up your mac - https://support.apple.com/en-ie/HT201250
2. Boot to internet recovery - https://support.apple.com/en-us/HT201314
3. Choose restore from backup, confirm erase
4. Wait