Arguments to commands need to be on the same line as the commands.
The "/S" is in the manual to indicate that the argument is a switch, i.e. something that is on or off and doesn't take any further details. You don't write the actual /S in the command, though.
If you use the menus in the command editor (Befehle, Argumente, etc. in your screenshot) then it's usually easier as Opus will take care of many of these rules for you. Check out the Toolbars 1 video in the tutorials section of the forum for some examples.
There isn't a built-in command to clear the clipboard but you could do one of these things:
[ol][li]If you've made a mistake, find the first file that you do want to copy and put it in the clipboard normally using Ctrl-C or Ctrl-X (which run Clipboard COPY and Clipboard CUT). After that you can use the other commands to put more files on the clipboard. They will "stack up" on top of the first file you put in.
...or...
[/li]
[li]Use my small SetClip tool and create a button or hotkey which runs SetClip.exe -e to clear the clipboard.[/li][/ol]
BTW, you can't "stack up" combinations of copy and cut. Everything on the clipboard must either have been cut or have been copied, and not a mixture of the two. (This is because the list of files is kept in a format compatible with Explorer etc. and that format doesn't let you mix operations.)
You can make a shortcut for directly emptying the clipboard. I recently started using the CLIPBOARD COPY ADD function, & this also well, except SetClip surely has some more useful features. What i´d like to see in Opus, btw, would be a way to show clipboard contents in a lister (i´m not sure, if maybe SetClip has a convenient way to show the contents).