I don't understand. In the hints I see: "my parents' wedding" as well as "the wedding of my parents". These are also shown as accepted translations. If you are referring to the usage of "parents's" to my knowledge that is only used for names and even then it is optional. (My spell checker right now is showing red for "parents's)
Jaye, in some of the exercises in this section there are "blocks" to choose - I don't know what you call them. But none contain the apostrophe: no possessive exists to choose a correct answer. Since I use only a computer and not a phone, it's also possible that I'm receiving something slightly different than those who use a phone for the exercises.