To create a talk, the Category and Speaker fields are required. Make sure you head to Content > Categories and Speakers to add that first.
Once the Categories and Speakers are set up, you can go ahead and create your talk.
Attendees can filter the event's schedule by category, so it's easier to add sessions of interest to their schedule.
It can also help when creating your tickets and restricting them by Categories.
Please read our article on how to create tickets for your event here.