"The bride is coming to the wedding on a truck."
Translation:הכלה באה לחתונה על משאית.
As I understand the English sentence, the bride is coming by truck (and not literally on top of a truck) - if it is so, then במשאית is great.
Perhaps it is possible to modify the programme by adding the acceptable correct alternative translations for all languages taught by DUOLINGUO. It may be difficult and costly, though, and it is a very good free service. Perhaps they have already forseen this, and that is why we are invited to point out the issues as we seem to come across them... It must be costly anyway. I must add that at leat in my case I often miss a little tiny detail that makes my answer wrong...