Hey doipyyy,
What's the Realm Name on the bottom right of the login screen of the WoW Client? If it's 'Test Realm...' than log in and hit the Enter key quickly as soon as you see Success then change the Realm to Nostalrius Begins.
Did you reinstall using the
Nostalrius Installer?
If you're installing to C:\Program Files (x86)\World of Warcraft Classic\ than reinstall to C:\World of Warcraft Classic\ which will reset all settings and fix common permissions issues.
Edit: It may be a temporary ban from too many failed login attempts to the Test Realm which is why you see 'Disconnected' then after a few attempts it says banned. I'm not sure how their IP bans work or if they're even utilizing them but that may also be a possibility.