This message occurs if the user tries to change the use of the drive too soon after the drive has been inserted or the enclosure has been connected for the first time. It can also occur because of a hardware fault or if the user tries to change the use of a drive that has been removed from the system but is already managed.
Wait and try again. If this doesn't work after 10 minutes, replace the drive. A drive that has been removed can still have its use changed to unused.