"Why can't you cook?"
Translation:Dlaczego nie umiecie gotować?
Yes, "umieć" is a modal verb, so it needs infinitive. In a way, that is not so different from English:
- I can cook
- You can cook
- She can cook – and not 'She can cooks' ;-)
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?"
My understanding (and I stand to be corrected) is that "umie" means more "knows how to" while "moze" refers to physically being able to do something at a particular moment.