Translation:In that country there is no freedom of expression.
It appears DL wants the phrase "libertad de expresion" to be translated as the common English phrase "freedom of speech" (which is an iconic phrase in American English), rather than translating "expresion" literally as the English word "expression." However, either translation would be understood by an English-speaking person. See http://www.spanishdict.com/translate/libertad
Liberty and freedom, yes, are synonyms--in English. However, Duolingo is looking for the translation of "freedom of speech" or "freedom of expression," as that is what would be most relatable to English speakers.
This is one of those questions where DL is asking for the most cogent interpretation of the Spanish phrase, not necessarily the exact translation.
eagersnap's post below is very helpful. Nice link, TilEulenspiegel--SpanishDict is a reputable source. I use it often.
It sounds very unnatural. I would say that liberty in English is an abstract term that is not applied to specifics like speech, choice, thought etc. Those would be coupled with freedom.
I would not think of libertad as = liberty. Rather I would consider that libertad covers both the meaning of liberty AND the meaning of freedom in English.