You need to add scriptcs to you PATH environmental variable, otherwise the system cannot find the executable.
The process is described in the ScriptCS documentation:
For simplicity, we will be using the Chocolatey Nuget method of
  installation. Copy and paste the following command into your console.
  This will execute a PowerShell script that will download Chocolatey,
  place it on your local drive, and configure your path. Make sure to
  reload your session so your new path is loaded.
@powershell -NoProfile -ExecutionPolicy unrestricted -Command "iex ((new-object net.webclient).DownloadString('https://chocolatey.org/install.ps1'))" && SET PATH=%PATH%;%systemdrive%\chocolatey\bin
Next execute the
  following command in your console. This will download ScriptCS, place
  it on your local drive, and configure your path. Make sure to reload
  your session again so your new path is loaded.
cinst scriptcs
If you prefer adding scriptcs to your PATH manually (or simply want to get a better general understanding of PATH), have a look at this excellent answer.