"I need some pretty dresses for the party."
Translation:Yo necesito unos vestidos bonitos para la fiesta.
i did the same without "yo" and it was accepted. it should definitely be accepted with "yo" included though.
A copy-paste of my answer which was accdpted:
Necesito algunos vestidos bonitos para la fiesta.
They must have fixed it.
Unos means some (masculine) while usar is to wear or to use. I think it does not use usar because the original sentence does not talk about wearing dresses.
A matter of "I need to wear some pretty dresses for the party" vs "I need pretty dresses for the party."