The use of the periphrastic possessive (using several words like, "of my--X") to indicate possession is entirely correct. The other possessive--by inflection (using an apostrophe-s) is also entirely correct. For inanimate objects English speakers generally use the "of the" form for example, "the side of the car" instead of "the car's side". or "the effect of the wind" instead of "the wind's effect". Both are equally correct though. When it comes to animate objects we usually go the apostrophe + s route. They are all correct, but Duo seems stuck on only one pattern.