I was the one that spotted that it is not there. Read back a few posts. But the area is pretty greasy under the top clamp so maybe it is there only covered with grease. It's pretty hard to imagine the bike being rideable if the slotted nut is missing, isn't it? But I can't see it. And I can't see the threads either so maybe it's covered with grease.
If it is there it needs adjustment and possibly cleaning/greasing. The bearings are supposed to be cleaned and greased once in awhile. Some riders do this every year which I don't really think is over doing it but is more than I do it. I'm just a little lazier than most.
He's got the top nut off already. It was loose which is what started this. A 36mm socket is fine but doesn't fit in the tool roll very well especially with the 1/2" driver. There is a 36mm box end Suzuki or Kawasaki wrench that is much heavier than our Dog Bone wrench.
I'm pretty sure you will have to take the top clamp off and look/see what is there. Then clean/grease both the bearings. Simple.
Never memorize something you can look up.