"I cannot do it."
Translation:Nie mogę tego zrobić.
What is distinguished in English as perfective/imperfective, is called in Polish dokonany/niedokonany. Roughly translating: accomplished/not accomplished. Zrobić is 'accomplished' - you did it, finished the job. Robić is not. By logic, you cannot use verbs that are 'accomplished' (perfective) in present tense.
Here, the precise difference would be:
Nie mogę tego zrobić (you asked me to do something, and I cannot do it. by 'do it' we understand accomplishing it to the very end)
Nie mogę tego robić = I cannot be doing that. For example, a man cheating on his wife, who suddenly realizes that he loves his wife and has to stop the affair, can say to his lover: Nie mogę (dłużej) tego robić = I cannot be doing that (any longer). OK, that would probably still be simple 'do' in English, but I hope it helps.