Is RXTT an OS component since eCS 2.x?
In general: if so, and HELP 1003 wasn't some clue, then go to a prompt near you and type: view rxtt36 file and directory names.
No quotes in the OS/2 command (a.o. the PARAMETERS-setting of the WPS object), no PARSEing of quotes, no quotes when executing an external OS/2 commands.