Error: -69711 this operation couldn't proceed because the target's boot helper was mounted; you may want to try again
This happens when you've booted to the recovery system on-disk (CmdR) instead of using Internet Recovery (OptionCmdR). Since the local recovery partition is associated with the stack you're trying to resize, and that partition is mounted, diskutil refuses to perform the operation.

(This may only apply to FileVault 2–encrypted volumes, which require a working recovery partition to boot, but I don't have an unencrypted computer to check with.)