Sorry for the dumb question, but I created the new category, but when I open the forum and I click over to me this error "The requested URL /forum-2.html was not found on this server."
How can I do?
your forum root folder has a file named htaccess.txt rename it as .htaccess (it has a dot at beginning)
or navigate to below location
admin panel >> configuration >> settings >> Server and Optimization Options --> Enable search engine friendly URLs?
select disabled and save the settings.