PHILPPA --
Are you actually asking about Project for the Web or Project Online? To know this for sure, does the URL of your web instance include "project.microsoft.com"? If so, you are referring to Project for the Web. And in that case, the application DOES NOT include the Network Diagram view, and you can only display the Critical Path by filtering the tasks to see only Critical tasks.
On the other hand, if you are using Project Online, then you will need to use the Microsoft Project Professional desktop app, connected to your Project Online instance. In Microsoft Project, you will automatically have a built-in Network Diagram view, and there are multiple methods for displaying the Critical Path, including applying the Tracking Gantt view.
Hope this helps.
