Powershell: Permanently Modify Path, "Requested registry access is not allowed."

It sounds like you might not be running as an elevated admin. By default, I believe there's a PowerShell shortcut on your taskbar in Server 2012. Right-click on it, and choose "Run as Administrator" (or something like that). Then try running the command in your original post.

Give permissions to HKLM\System\CurrentControlSet\Control\Session Manager\Environment to a desired user