Role Management Tool to Install .NET FrameWork 3.5

You get this error when Installing SQL Server 2012 Enterprise edition. 

Because SQL Server 2012 ships with .NET 4.0 Framework. 

And Windows 2008R2 already has 3.5 inbuilt you just need to enable the already existed feature within WINDOWS 2008R2 from server manager as shown below. 

 

Server Manager Enable .NET FrameWork 3.5.1 Feature. 

Right click on computer and select Manager and browse to Features as shown below. 

Once selected this feature and continue SQL Server 2012 Installation it will be go smooth.