WHMCS documentation describes creating categories under Support > Knowledgebase using Add Category, then creating articles using Add Article, assigning a category, entering contents, and saving changes. The rich text editor or HTML view can be used for article formatting.
Official reference: https://docs.whmcs.com/9-1/support/knowledgebase-tutorials/
Knowledgebase suggestions can be enabled in General Settings > Support. WHMCS documentation states that ticket suggestions require at least five articles containing at least 30 words each; this starter set is designed to exceed that baseline, but the feature should be enabled and tested in your installation.
