Yes, it can, and conor.raff is dead wrong. There is no such thing as one dialect or another being more or less correct, that's what makes them dialects (unless you're such a stuffy prude that it's bordering on xenophobia, which to put it mildly, isn't the Duolingo way). What makes a dialect a dialect isn't just having an accent, it's having your own variation on grammar and spelling rules that is just as rigorous and standard as any other.
British English or American English? Both are standards in one nation or another, and taught as standards to several million children year after year. It's not like the quiz rejected "I'mma be at that garden out back", it rejected a perfectly clear and concise standard American sentence, one that could very well show up in a novel, script, or even textbook.
Well, there is a variation: "The garden is in the back of house", where in the USA "back" is used to mean the whole area to the rear of the house. But even that usage is not Middle English.
You hear "X is in (the) back of <Y building>" on US TV shows and films but that doesn't mean it is good usage.
"The garden is in the back of the house" tells us that the garden in inside of the house, in the back part of inside the house. "The garden is in back of the house" tells us that the garden is behind the house, in the back yard. I keep repeating the "strengthening places" and keep getting this answer wrong because it does not sound natural to say "The garden is in the back of the house".
I thought I've already added it after someone's comment... okay, added now.
Even if "in the back" technically means "inside of the house", this is a nuance that surely will not be known to many non-natives, so it's going to stay.
EDIT: And actually your answer will be the default version now.