DUO, a facility to look up the verb table alongside would be useful.

Whats the difference between mają and ma?

But that means cookie but says cookies

I've written ,,On ma ciasteczka'' as a correct answer. But it is not. Take into account that the word "ciasteczka" is singular and means "a biscuit/cookie". "Ciasteczki" is the correct plural form and that means "biscuits/cookies".