You obviously removed the bolt that clamps onto the splines, right? I don't see it in the pic so I think you're good, but it has to be removed completely, loose won't do it. Sometimes there just a lot of grit in the splines so you have to work on it. Or use a screwdriver to carefully open the split a bit.
If it's rusted or something maybe a penetrant would help? PB Blaster?
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1978 Honda CB550K