This error indicates that Tex2RTF has not implemented a standard ATEX macro, or that a local macro package is being used that Tex2RTF does not know about. It can cause spurious secondary errors, such as not recognising the end document command.
You can get round this by defining a macro file (default name tex2rtf.ini) containing command definitions, such as:
\crazy [2]{{\bf #2} is crazy but #1 is not}
\something [0]{}
\julian [0]{Julian Smart}
New commands may be defined in LATEX files, but custom macro files will have to be defined when local style files are being used.