You were probably a bit skip-happy during the tutorial and missed over the fact that important clients need to be finished before certain chapters :)

Go to Design and then click Clients. You'll notice there's an important client task for you, which is to create a yellow sundress.

Your job is to buy the necessary missing parts and to recreate the dress exactly. Once you've finished this client, you'll be awarded with the yellow sundress outfit, and you can wear it by going to Outfit and clicking the category outfit, it should be in there :)

I did the client but it was i have to wear a yellow sundress.. i completed the client but still wont let me move on

There is no yellow sundress in the outfits category, only kayla vu and aeantiz. ???