Please follow the steps below:
1.Please navigate to the Dashboard > Forums > Settings > General admin page, make sure the "Forum Page ID" is exist, click on "Update Options" button.
2. Deactivate and activate the plugin again.
3. Delete all cahes and press CTRL+F5(twice) on the frontend.
4. Navigate to the Dashboard > Forums > Dashboard admin page, click on "Delete User Cache" button.
5. If you have a cache plugin, you shouid exclude wpForo Forum page from your cache plugin, before checking. Also please read this FAQ topic about the wpForo and Cache Plugins: https://wpforo.com/community/faq/wpforo-and-cache-plugins/