If you are intending to use AstroImageJ, that is a different build from
ImageJ. You need to download it here:
http://www.astro.louisville.edu/software/astroimagej/index.htmlThe only time I have seen the message you mentioned is when trying to
open a Multi-Extension FITS file. AstroImageJ supports only single image
FITS files, but can load many single image FITS files at one time into a
"Stack".
Karen