Screens make colour by adding red, green and blue light; presses make it by laying four inks on paper. A file saved in CMYK describes ink, and a browser that expects light has to convert it. The result is often dull or shifted colour, and some systems cannot read such a file at all.
CMYK files usually reach a webshop from print work: a catalogue, a packaging design, an agency export. Several marketplaces state that they do not accept the mode, or warn that it causes errors, and ask for RGB or the sRGB profile. A few specifications written for print and retail suppliers still accept both.
Converting to RGB in an image editor before upload solves the problem, and it is worth comparing the colours afterwards, since strong blues and greens change most. Photos from a phone or camera are RGB from the start, as are images produced for the web.