If the Lightsaber says "Main Hand" only, it won't work.
You need one that says both "Main hand" and "Off Hand", or only "Off hand"
Also, just checking (you never know), you're not trying to equip a double bladed lightsaber in the offhand are you?
How do I get such a lightsaber? Do I have to buy it? If so, where are they sold? Why didn't I get such a lightsaber in my Sentinel bag when I chose that class?
Thanks everyone for all your help.