I forgot to put a 'the' at the beginning, which would still have made perfect sense, yet duolingo thinks it make a whole big difference, and said its wrong. But it doesn't even make sense! shouldn't it be 'ההר הבית'? because 'ה' is a prefix for 'the'?
Here הר הבית is in smichut (construct state). Words in smichut, when definite, get ה only before the second word of the pair. ההר הבית is incorrect. The same like saying school is בית ספר and the school is בית הספר.