minecraft you not on the white list ?

Answer 1

the whitelist is a list of people allowed to join the server and play on it

if your not on the list talk to the host or one of the admins, if you dont know any then youll need to look for another server

Answer 2

If you are not on the White-List on a certain server (E.G: Jujucraft) then message or contact the host or admin