In order for the sentence to make sense in English, there has to be an object. If Portuguese doesn't need one, then it's useful for the English speaker to know that. So Duo should not accept the literal sentence and require that we understand that "it" is understood in Portuguese and that we have to express it in the translation. Doesn't that make sense?
I'd like to defend the Portuguese sentence. Using the past tense to say you like things is very common in Brazilian Portuguese. So if someone asks you if you like something you could say "Gostei" (past tense without mentioning the thing you like). So I guess this is the way someone would respond if they found out you had subsequently changed your mind "Você disse que tinha gostado" and the Portuguese sentence is something you might hear.
Nothing really. It's colloquial, and we all use it, but, if you are a stickler, in reported speech: (1) you said "I like it" and (2) I am reporting that fact, so your statement should be in the past perfect tense as the first past action. My reporting of it is the subsequent action in the past -> simple past.
As you say, the English sentence really does need an object. A possible problem with adding your sentence to the pool comes when it must be translated to Portuguese. Most people, me included, will try to translate your "it" to a word like "disto" which is not present in the original Portuguese. This is just one of the difficulties Duolingo faces when trying to present vernacular sentences in both languages.
The sense, then, is "You used to say you liked it," for which "[But] you said you liked it" also expresses that idea given the right context. I guess this sentence means you once said you liked it, so maybe you still do or maybe not. Now, if we use a verb of action rather than feeling, the perfect subjunctive translates just that way in Enlgish: "You said [yesterday] that you had enjoyed it [last week]." So the problem is trying to use the verb "like" in this context.
I don't know if I understand your doubt, but:
"Você disse que tinha gostado." - "You said you had liked it."
"Ele disse que você tinha gostado." - "He said you had liked it."
"Ela disse que você tinha gostado." - "She said you had liked it."
"Ele disse que tinha gostado." (or "Ele disse que ele tinha gostado.") - "He said he liked it." - you can suppress the second ele - pt
"Ela disse que tinha gostado." (or "Ela disse que ela tinha gostado.") - "She said she liked it." - you can suppress the second ela - pt
"Ela disse que ele gostou" - "She said he liked it"
You're welcome ! suppress pronouns it's not an easy job.
Some phrases fits better with the pronoun suppressed. One pronoun that we usually suppress it's Eu - I
Eu gosto de correr - Gosto de correr - I like to run
If you need help in Portuguese you can ask me. I need help in English, so we can help each other