The most common source of this issue is a missed level somewhere.You must complete each level with a letter grade, except for the Encore levels (the last three levels after the two Finale levels).
The easiest way to check this is to load into a Story game with a controller connected, and make sure you set up the controller and keyboard as Player 1 and Player 2.
Then, walk in front of each door in the Chapter lobby (the doors that say Chapter 1, 2, etc...). You will notice a little panel with colored bars to the right of each Chapter door.
Make sure that all of the bars except the last three have some color in them. The color in the bars corresponds to your earned grade in that Chapter. If you are missing color in any of the bars, that means you accidentally skipped a level in that Chapter.