"The girl puts on a white dress."
Translation:La niña se pone un vestido blanco.
Like this? [La niña ponerse un vestido blanco.] This sentence is wrong because it doesn't have a conjugated verb. Since the subject is the girl, the verb has to be conjugated in the 3rd person singular, the way it is in Duo's sentence.
You could use your ponerse if the sentence already had a conjugated verb first:
Ella va a ponerse un vestido. She is going to put on a dress.
Ella quiere ponerse un vestido. She wants to put on a dress.
Ella necesita ponerse un vestido. She needs to put on a dress.
Well I'd actually meant keeping poner conjugated to pone, and attaching the pronoun to the end of it, but it has since become apparent that I can't do that with a conjugated verb and I must put the pronoun in front. However, thank you for the response, it is informative and well stated.