Kami datang besok. Napa salah ?
datang = come || arrive = tiba, sampai
'will', 'going to' ? there must be something wrong in this lesson
"going to" is expressed to say you go to a place, but it's also used to express the future. I'm going to buy fruit = future (near) = Aku akan membeli buah (waktu dekat).
Nothing is wrong. It implies the fact that "we" arrive tomorrow.