In Spanish it is common to omit the pronoun that is the subject of the sentence. The subject is understood from context and verb ending, but isn't needed in order to have a complete sentence.
"Entiendes" = "You understand"
"Entiendo" = "I understand" "Entiende" = "You [formal] understand"
In English you can't do that, probably in large part because our verbs don't show nearly as much variation - take away the pronoun in the three examples, and the English versions are identical.
So the correct translation or "Require una cita." is "He [or SHE or IT ] requires an appointment." Complete sentence.
You wouldn't use the 'they' form, it's being used as a third person singular pronoun so you'd still use requiere.
Personally I think it should be accepted, Duo leans toward natural English and 'they' is very commonly used, so it's inconsistent to suddenly require more formal phrasing. I get that people could mistranslate the Spanish verb as being plural and still get it right, but that's just a quirk of how English has developed over the centuries, and people shouldn't be punished for it.
Report away ;)
You mean the una? That's 'a' in Spanish (for feminine nouns, it's un for masculine ones). 'One' is uno.
Just so you don't get tripped up - 'a' and 'an' are both indefinite articles (as opposed to a definite article, which is 'the' in English, that's used to identify a specific, definite thing). 'A' and 'an' are the same word basically, 'an' is just a variation that makes it easier to say the next word if it starts with a vowel sound. "A apple" is hard to say! Anyway, point is they don't both have an equivalent in Spanish, so don't worry about that.
The indefinite articles in Spanish are una for feminine nouns, and un for masculine nouns - not uno! (I got tripped up by this too.) Uno means 'one', and in Spanish you don't say 'one car', you say 'a car' (which is the same thing if you think about it). So it's always un coche, never uno coche.
You can use uno as a noun in basically the same ways we use 'one' in English. So for counting (uno, dos, tres) and phrases like tengo uno - "I have one", and some fancier uses. Don't worry if it seems a bit complicated at the moment, just remember to use un or una (or unos/unas for plurals) when you need to say 'a', and don't say uno (thing)
'She' works too. The translations they give are usually just examples of possible answers, because of the way each language works!
Like for example, he/she/it/you (polite) all work as subjects for requiere. You need to keep that in mind later when you get multiple choice questions, it's all about learning to think in the language.
Ok, so there's typically two places you'll see se used this way - with reflexive verbs, and in impersonal statements.
Reflexive verbs are the ones where the subject and object of the verb are the same - where someone does a thing to themselves. Like me llamo, te llamas, se llama - the pronouns always match the conjugation of the verb (I call myself, you call yourself...) and the se is just the third-person pronoun. Have a read up on reflexive verbs if this isn't familiar
It's more common to see things like se requiere, which doesn't mean 'requires itself' - it's an impersonal form, and you'd translate it as 'you require' or 'one requires' or something like that. It's not 'you' as in you, it's 'you' as in 'people in general'. You're not talking about anyone in particular requiring an appointment, so you use se to make the verb impersonal.
That's the difference - your Google version says 'you need an appointment', as in people in general. The Duo version (requiere una cita) isn't impersonal, so it specifically says he/she/it needs an appointment, or you need one (polite usted form). See the difference? The English version is ambiguous here, the person speaking could mean it either way, so both Spanish translations are valid
Here's some stuff on lo, but you rarely use it when 'it' is the subject of the verb: