Mosty likely you've used a PDF preset that specifies conversion to CMYK, but your transparency blend space (Edit > Transparency Blend Space) is set to Document RGB.
Tell us more about the file, and where it's headed.
If I think right this normally comes up when doing [Smallest File Size].
Ive always understood this to mean that its trying to use a different colour space than what your document is set up to - usually a smaller profile to minimize file size.
You can either match you document to the output, or match your output to the document. If your going for a small size, then match your document to the output.
Personally I just skip this as its never effected my work.... to what ive noticed.
Yes this was just to send my PDF to a client to view.
I want the final output to convert to cmyk for printing
But I will use the press quality preset
Sent from my iPhone
Waht setting did you use to make the PDF to send to the client?