why do we absolutely need của in this sentence. yes I know it indicates possession but in other exercises we've got away with nhà tôi bố tôi etc. So is this case different or just not been loaded in the database yet?
Maybe it was not handled by the mods. It is totally normal without của.
I've only ever seen của left out when it was referring to a family member. Mẹ tôi for example.