Installing on Windows Server 2012 R2 often fails because the source files aren't kept on the local disk to save space. The standard way to fix this is by using the microsoft-windows-netfx3-ondemand-package.cab file, which is found in the \sources\sxs folder of the original Windows Server 2012 R2 installation media. Option 1: The Quick Command Line Fix
Many legacy applications (e.g., SQL Server 2014, Exchange, custom ERP tools) require .NET 3.5. On Server 2012 R2, the default “Add Roles and Features” wizard tries to download files from Windows Update, which often fails due to: Installing on Windows Server 2012 R2 often fails