dialabrain:
I documented my steps ad nauseam on my link
including Hardware, OS versions, screenshots, steps
I am not exactly sure how you got to Software Update from what different method on the link Limnos posted -- or what hardware/OS you are on in that screenshot -- but I think if you were after the actual download (not an updater from a working OS automatically installing the update) it would take you to the
"Update not found
The requested version of macOS is not available."
that I and others have found
I do recall having Upgrade and Download options using various methods but in every case it ENDED in not available or not on server, except for the one scenario noted on my earlier notes ad nauseam
I would be more interested in seeing the end result -- the installer icon in applications (that I can use to make a USB boot installer) -- what hardware and OS version you are on -- the method and steps you took -- a screenshot can tell (me) a thousand words -- your SS doesn't really tell me anything (no disrespect intended)