Explanation/Reference:
Explanation:
Configure Policies for VPN Enforcement
The NAP health policy server uses the Network Policy Server (NPS) role service with configured network policies, health policies, and system health validators (SHVs) to evaluate client health based on administrator defined requirements. Based on the results of this evaluation, NPS instructs the virtual private network (VPN) server to provide full access to compliant NAP client computers and to restrict access to noncompliant client computers when NAP is deployed using full enforcement mode.
Before performing this procedure, you must install a certificate for Protected Extensible Authentication Protocol (PEAP) authentication. For more information, see Install a Computer Certificate for PEAP

You cannot continue without a valid certificate:

References:
http://technet.microsoft.com/fr-fr/library/dd314165%28v=ws.10%29.aspx