Negation has two components. You need both. "Ne" tells you that there's a negation. The other tells you what kind. In this case, "pas" means not.
Actually, with the verb "pouvoir" you can sometimes omit the pas : 'je ne peux faire cela' is correct. However, I would never say "je ne peux" alone without a complement.
Perhaps your "last question" did not have a verb in it. Something like "pas elle" or "pas trop." Negation of the verb requires both components (with some exceptions for special verbs).
Follow the process of the exercises. While you may be correct, to teach "I can't" before "I am unable" seems perfectly logical to me; "I can't" is very basic, simple, and commonly said. Teaching is often taught in a linear fashion; basics first.
Merci beaucoup! For all my pontificating in these forums, some really small though important things just don't get through to me right away. I'm going to write out sentences using most and study them. I encourage others to do the same.