How to grant privileges for users domain conect to VPN server?

Hi everyone,

   My system used to AD Windows server 2008 to manage. Current, i want to grant privileges for users domain can connect VPN but i can't. It notify error:

NOTE: FlushIpNetTable failed on interface [12]{49D27953-...} (status=5): Access is denied
.....
ROUTE: route addition failed using CreateIpForwardEntry: Access is denied. [status=5 if_index=12]
..

 Althought, I added users into Configuration Network Group and create GPO allow access Network Connection.

Please, help me!

Thanks,