The error code is server-side, so it is server-specific. However it usually means that the remote shell cannot find the scp command to perform the transfer. Make sure that scp is in PATH. Note that the environment for WinSCP may differ from the one you have when logged in using terminal. Execute remote command echo $PATH to see actual PATH for WinSCP session.

Also it is worth trying to use SFTP protocol instead of SCP, which is a much better protocol.