Building on this @MohamadAlhussein, I have a follow-up question. Although I have set my first milestone/task as "complete" and the Next Bill button is visible, when creating an Invoice no Items are added. I have the same billing schedule set up. Do you know what could the reason it went wrong?

Thank you for your response @Elychelle Gulen-Oracle! Indeed I've added the Item manually myself when creating the Invoice. When I pressed the 'Next Bill' button from the Sales Order, the item did not automatically load itself. I've ran into this problem multiple times, also with other Sales Orders. Would also perhaps know…