Well, actually carta in Spanish doesn't mean card but letter (meaning a post letter) or sometimes menu (alongside menú). Card is rather tarjeta: credit card, tarjeta de crédito; birthday card, tarjeta de cumpleaños, etc. If maybe in some countries you find carta used as card it is probably because of the influence of the English language.
Rachel996051, your sentence ”You do not write the book” can be translated in Romanian better as ”Tu nu scrii cartea.” (this book).
Also, the Romanian sentence presented in the exercise is at the present tense, with the meaning ”you are not the writer of a book, you are not involved in writing it”
while your proposal ”You did not write a book” is at past tense, and can be translated as ”Tu nu ai scris o carte”.