Client certificates were detected on the report server computer
Upgrade Advisor detected one or more client certificates on the IIS Web site that hosts the report server or Report Manager virtual directories.
SQL Server 2012 Reporting Services does not support the use of client certificates to authenticate users. Upgrade can continue, but client certificates will not be used by the upgraded report server.
You will have to use a separate solution such as ISA Server to ensure any client certificate authentication requirements are addressed.