command line error "no volume specified"
The bless command has be deprecated. Basically, the command is not compatible with APFS. Other replacements are:


The systemsetup command. Use options -setstartupdisk, -getstartupdisk and liststartupdisks.
The Mac Startup Manager. Basically hold down the option key at startup. To make an OS the default, hold down the control key before selecting.
The Startup Disk pane in the System Preferences application.
The third party rEFInd Boot Manager. This can be installed without disabling SIP.


However, I you still want to use the bless command, then syntax is shown below.

sudo bless --device /dev/disk0sX --setBoot


Where positive integer X can be found by entering the command given below.

mount | grep "/Volumes/ML2 "


I tested this command by boot to Yosemite on a 2007 iMac. I used the bless command to select El Capitan as the default operating system to boot.


  Note: Yosemite does not use System Integrity Protection (SIP)


If selecting a BIOS booting Windows (or other) operating system, be sure to include the following option.

--legacy


Below is the result when trying to use the bless command from High Sierra with SIP enabled. The machine is a 2011 iMac.

Marlin:~ davidanderson$ sudo bless --device /dev/disk0s6 --setboot
Could not set boot device property: 0xe00002bc


Below is the result when trying to use the bless command from El Capitan with SIP enabled. The machine is a 2007 iMac.

Steelhead:~ davidanderson$ sudo bless --device /dev/disk0s5 --setboot
Could not set boot device property: 0xe00002bc