I assume you have created top level categories without child forums in them. If a category (blue panels) does not have forums (gray panels) it will not be displayed on the front-end. Each category should contain at least one forum.

Please read the top information box in Dashboard > Forums > Forums page.

Also, please make sure your Forum page is excluded from cache plugin, check this topic about cache plugin: https://wpforo.com/community/faq/wpforo-and-cache-plugins/