I was trying to link a series 5 to a iPhone 12 and had this exact issue.
The watch was a "replacement" watch that was sent to me and I sent in an series 5 for repair. I keep the replacement watch.. which I am assuming is not "new" but a refurbished watch.
I tried everything listed here. EVERYTHING.
I did chat support, to no avail.
The Apple Support person said that I needed a new watch. Confirmed that there was a software issue..
chilegirl
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