The critical path marks activities and milestones, which in case of delay immediately lead to the exceeding of the planned project end.
You can display the critical path only in an already existing planning.
For more information on how to create a planning, see the category Create planning elements.
Open an existing project or create a new one.
For more information on how to create a project see Create a project.
On the View tab, in the Details group, click Show Critical Path.