Because you are using "cannot" in your translation. You are adding a verb.
I remember = Ricordo.
I do not remember = Non ricordo.
I can = Posso.
I cannot = Non posso.
I cannot remember = Non posso ricordare. (Literally: I cannot to remember) Someone correct me if it's not supposed to be the infinitive in the last case. I'm just learning that one.