[duo797]

Hatsuto is pretty right on this one. Don't think too much into this phrase, just use it as a set. the しなければいけません, しなければなりません have the set meaning 'must not do (verb)'. So 行かなければ行けません is 'Must not go'. Also, it's not so much that the negative verb becomes positive in the english translation. You have two negative verbs, (しなければ, なりません) acting as a double negative, which makes it positive. But yeah, the long and short of it is just look at these verb patterns (especially for prohibition and what someone must do) as set phrases with set meanings.