When you install TMG Reporter for the first time, it first detects if .NET 3.5 is installed, and if not, will attempt to activate the feature.
If your installation of Server 2012 does not have .NET 3.5 installation files available, this part of the installation will fail.
To fix the issue:
- Go to Server Manager and activate that .NET Framework 3.5 Feature.
- During the installation, you may see the message:
Do you need to specify an alternate source path? One or more installation selections are missing source files on the destination...
- Insert your Server 2012 installation media disk then click 'Specify an alternate source path'
- Specify the path to the sources\sxs folder in your installation media (for example D:\sources\sxs)
- Click Install.
This should install .NET 3.5 successfully and you can now re-try the TMG Reporter installation.