Choose from the menu Tasks and Task Capacity:

Expand your project so you can see all unassigned and unscheduled tasks:

Click on the one of the tasks and assign a resource who is absent during the assigned schedule. Once the task is saved, a conflict with appear:

To reassign the task to another resource or reschedule, click Cancel. To ignore the conflict, click Save anyway.

Related topics:

Absence Calendar

Absence during assigned task in a project

Absence in the Workload Sheet