That's future sense, isn't it? We haven't gotten to that yet, Duolingo!
Not to mention, wouldn't the correct translation for "Vi ska gifta oss" be "We are getting married"?
In the incubator it looks like this sentence is taught in Future, Lesson 2, which seems reasonable. Where did you get it?
Is vi skall gifter oss, also correct?
Sorry, I meant "gifta" not gifter.
Yes, Vi skall gifta oss is OK too. skall is a bit formal so you run the risk of not having it accepted just because it hasn't been added yet (I added it here now). I'd recommend writing ska in all sorts of texts though (even formal ones).
Would "ska" come off as more informal/less serious in really professional environments? Or is "skall" more old-fashioned and it's not really about formalities?