"They are not a couple."

Translation: One nie są parą.

[stramner]

Assuming this is male and female pair, I use oni, correct?

[Jellei]

Correct.

[tomboysquirrel]

What I say to Reylo shippers

[Rimdus]

Parą is similar to "pareja" in Spanish. Another word more easy to remember. Great. :)

[Gaiajack]

Why is "para" not in the genitive? I thought you used the genitive when negating?

[Gerardd88]

The construction here (a copular sentence) is "... jest/są + instrumental", for example "He is a worker = On jest pracownikiem (instrumental of pracownik)". Here, parą is the instrumental of para. Only accusative (direct object) changes to genitive when negated, other cases don't do that.

[iso174941]

Thank you Geraredd88 that clarifies something that was puzzling me

[Rimdus]

I meant easier. Sorry. XD

[Joe19902]

Would "To nie para" also work in this instance?

[Jellei]

I guess it can, for example "He is a rich man" can be translated as "To bogaty człowiek" (although it's never the preferred Duolingo translation), so your variant can work here as well. Added now. Probably not many sentences here accept those, so trying them may mean you get failed a lot.

[Joe19902]

Could "Oni nie są parą" also be translated as "They are not steam"?

[Jellei]

Assuming that you do not seriously ask for it to be accepted, I can say that this translation is technically, semantically correct, although of course rather nonsensical.