Hi, i am installed my forum on sub domain like forum.mysite .com but at this url google not accept google says URL must not have a path (example.com/path) or subdomain (subdomain.example.com) not accepting my sub domain i see many site are running on sub domain and they using long time adsense ho do they ? is it possible ?
The simple workaround is to have content on your main site to support the forum, get that looking good and then apply for adsense for the main site. Once that's up and running put the code on your forum. That's assuming that you own the domain and that @robyries hasn't pointed out your #1 problem of having a forum on a subdomain of some other website that you don't control.
Apply adsense for your main account after approval, add your sub domain to trusted domains list. Place the code there.