Boot Camp failed to uninstall Windows 10

I used Boot Camp to install Windows 10 on my MacBook Air, deleted it afterwards using Boot Camp but it said something went wrong and that i should look at it through Disk Utility.

Now i have a partition titled "free-space" of 45.1 GB that every time i try to delete it says that the operation failed.

Can someone please help me, "free space" is holding most of my unused space hostage.