Firstly, you need to make sure you've correctly set up a pay as you play fee in Payments Portal. You can either access the 'get team to pay' page, as a manager or coach, by clicking straight to it after submitting your match report...
OR by heading to your 'Payments' tab, then 'Pay as you play', then selecting the fixture.
Both methods will lead you out on to this page.
Click 'get team to pay'. This will release a notification to all of the players that played informing them it's time to pay for the game, and will present you with a screen that keeps track of who has and hasn't paid.