Are you trying to install Telnet on Server 2012 with Powershell? Simply follow the guide below and you'll have telnet up and running in no time. Installing this via Powershell is way easier than any other methods you may find online.
Simply open an elevated Powershell prompt and enter the following command:
install-windowsfeature "telnet-client"
Once the above command has been run, the telnet client will be installed on your Server 2012.