What do you mean by can’t complete the sets? How is your workout structured?

Typically you would choose a number of sets, each with a number of reps. Say 3 sets of lunges at 8-12 reps. You choose a weight such that you can complete at least 8, and keep pushing each week until you can do all three sets at 12 reps. Then, add a little weight and start the process over.

If you can’t do bodyweight lunges, that’s okay! You want to progress other exercises until you can manage it, perhaps bodyweight squats, then goblet squats holding 10lbs, then eventually the bar and however much weight you can add to it.

For youtube in general I recommend Jeff Nippard. I don’t know of a resource that’s dedicated only to barbell but the barbell is pretty much the foundation of working out, most things can be setup for it