Sounds like apache or some other web server might already be running on that port.
ps aux | grep apache
how do i attach a sub domain to my forum
any please tell me the solution , is there nay alternative
Do you want the forum at the subdomain, or do you want the subdomain to be something else?
i want to attach the link of other domain running on mybb software to the section of nodebb forum
You can add a link to the navigation at the top in the ACP.
Yes you can add external links.