After all work required on all tasks is entered, click the Calculate toolbar button.
Color highlighting shows where there is not enough resource availability to satisfy the required work effort. In the example below, Pat McClusky is needed .4 in August on the B2 task. He has total availability for the project of .6 (per the detail row at the top immediately below the task Gantt bar).
The B2 cell is red because he is needed on the B1 task .4 in the same period; only 50% of the .4 requirement on the B2 task can be satisfied.
You can double click on a cell in the resource column to filter the sheet on that resource. This makes it easy to see where the excess work is occurring.
Modify and add assignments until you have utilized all availability. Enter the full work effort necessary for success on every task.