Rarher than expecting people to compose grammaticaly incorrect English sentences, couldn't the solution simply be changed to "of the moutains"?
Trailing apostrophes seems like a problem (bug) for Duo.
That's clearly the case in High Valyrian.
i wrote it down as shown above, but allegedly had a typo. "correct solution" omitted the apostrophe.
I think it should accept "They see the front of the mountains" since "sides" is usually an additional word we don't need to use here in English. Unless this is a better direct translation.