Explanation
Before scheduling a lead, a related resource requirement must be created.
Add a requirement view to the schedule board.
After creating a requirement for the lead, it will appear in the Open Requirements view in the lower pane of the schedule board that shows all open requirements related to any schedulable entity. This is a default view for resource scheduling.
However, the views in the lower pane of the schedule board are simply system views and can be edited and added to. It's common to add custom resource requirements views for work orders (or leads in our example).
You can create a schedule board view that shows only leads to be scheduled.
Reference:
https://docs.microsoft.com/en-us/dynamics365/field-service/schedule-new-entity