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.