I was facing this prerequisite validation warning/error (Microsoft SQL server Reporting Services Web Service) when I was installing the “Reporting services extensions” component in AX 2012.
The Reporting services on SQL Server 2012 have already been configured and the Web Service URL is http://Myservername:80/DynamicsAXReporServerVD
Navigate to this path DriveName:\Program Files\Microsoft SQL Server\MSRS11.MSSQLSERVER\Reporting Services\ReportServer\RSReportServer.config
and replace this link
with this one
(this link you will find it in Reporting Services Configuration Manager under the Web Service URL.