If your request to activate a segment got rejected, it could be due to a missing document or failure to send the application successfully. You will get to know about the reasons because of which your request got rejected on your email as well in the segments section in the profile tab.
Rejected segment activation request -
In this case, you will need to place your segment activation request again by clicking on activate segment on the above screen from your app.
You can check, the process of activating a segment in detail here.