After you apply for a collab, you're implying that you are interested in participating in the collab. Participating starts with ordering*!
When you are accepted for a collab, that means the brand hand-picked you from the pool of applicants! By not ordering, it sets the collab back so please place your order promptly :)
*Note: this only applies to collabs that involve ordering products outside of the app. If you chose products through the app before applying to the collab, then there is no extra step to place an order if you're accepted.