Stranded subclass

I got a question, I just got into Destiny 2 this year and I’m rather confused so I’m hoping someone could help me out. I’m wanting to get the subclass Stranded unlock for my character and I’m trying to start/finish the campaign for LightFall. I started one mission but it’s not letting me progress, am I missing something? Is there a reason why, am I missing a step? HELP