When I click Forum on my website, I want the sidebar to disappear as you can see in the image. Where can I do it?
This cannot be done from wpForo side, you should edit the forum page and check your page templates, if you find some template with "Full width" or "No Sidebar" choose it. Also, there maybe an option to disable sidebar. All depends on your theme's page templates.
Click the Edit Page on the top admin bar and try to find something to disable the sidebar.
I couldn't find the settings you mentioned.
Edit the page with the shortcode of wpForo. Go to Page Attributes. In there probably you will see Full width or other templates for THAT page.