Normalizer produces this message every time you start Normalizer or Demonorm. You may think that it refers to a problem with the input PostScript or Encapsulated PostScript file, but in fact it refers to the PostScript file startup.ps, which Normalizer uses to initialize the PostScript interpreter.
The startup.ps file does not contain a showpage operator. The showpage is a PostScript command that directs a printer to print a page with whatever is drawn to that page from the PostScript file. It will always appear in a standard PostScript file as part of the print workflow, but the startup.ps file is not intended to generate a print job. So you can disregard this error message when it appears. The message has nothing to do with an actual Normalizer PDF conversion.