The latest RazorEngine 3.4.0 NuGet package depends on the Microsoft.AspNet.Razor 3.0.0 NuGet package which requires .Net 4.5
If you wish to use RazorEngine with a version of .NET prior to 4.5, then you will need to use the RazorEngine 3.3.0 NuGet pacakage.
You can install this using the package manager console with the following command.
Install-Package RazorEngine -Version 3.3.0