Before starting to use the Knowledge base you need to create a new standard page called Knowledge Base or any other name you want the knowledge base to be called and insert the following short code: [basepress].
In this page you can place any content you like as for example an introduction to your knowledge base, with text images and any other element you need.
Just place the short code where you want it to appear in the page.
Once the page is ready publish it and add a link to it in your menu of choice.
This will be the entry page to the knowledge base.
Once the page and menu link are ready head to the Settings page and select the page you just created from the select menu and save the settings.
At this point you can start to populate the knowledge base with your first product, its sections and articles.