I recently deleted a partition and have 60GB of free space. I would like for my main partition to use that, but whenever I go to resize it, I get the error "You can’t perform this resize unless it has a booter".
This Ask Different post has what you're looking for. Don't go looking for an "answer," the author for some reason decided to write his own answer in the "question" post. The method worked perfectly (and non-destructively, but you should always take a backup anyway, just in case) for me!
Reboot in Recovery Mode by holding down Command-R as your Mac starts up.
Open Terminal (from the Utilities section of the Menu Bar).
Run the command,
diskutil cs list
Take the Logical Volume ID (a long string of letters and numbers) 10 lines up from the bottom of that command's output, and use it in the command:
diskutil cs revert [Logical Volume ID]
This should convert the CoreStorage Logical Volume to a normal disk partition.
diskutil cs listagain, and you should receive a "No CoreStorage logical volume groups found" message.
Now you should be able to resize your partition normally through Disk Utility.