VPN

I looked vpn client properties. in its dial in properties i allowed dial in.
but now also when i dial after comin verifying user name and password. the
following error come
Error 778, it was not possible to verify the idendity of the server.
i am a begginer and try to study ,myself active directory thats y i dodnt
know whether i miss some basics to configure vpn. please help me.
.

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