"Why can't you cook?"
Translation:Dlaczego nie umiecie gotować?
może is 3rd person singular form of 'móc', but you need 2nd person('Why can't you cook?' after all).
'ugotować' is a perfective verb form of 'gotować' so means not only cooking but also finishing doing it.
'Dlaczego nie może ugotować', would be something like: "Why can't [he/she/it] finish cooking?"