It seems like いけません would be a conjugation of いける (which presumably
would mean something like "to not be permitted to")
It's true that いけません means something like "can't do", that's why if you want to say "I must do X" in Japanese you need to actually reword it to "I am not permitted to not do X". Thats why to say "must" you use:
Negative te-form + 「は」 (...