I, alike many of the folks trying to update their watch. Left my watch on a counter dead for a good few months. I just spent about 4 hours absolutely stumped with this same issue and got to page 9/17 of the answers on here. My solution.. (not sure if you wearing the watch is related) I took it off the charger and put it on, both my iPhone 12 Pro Max and my Series 4 watch were fully charged. I went into my Xfinity app and made two separate networks 2.4, and a 5gH. I connected my phone to the 2.4, and turned off cellular data.... IMMEDIATELY popped up ready to download. I shut both my phone off, watch off, uninstalled/reinstalled the app, signed out of iCloud. If it was a suggestion... I tried it. Hope this helps! Its currently downloading, says it has about an hour! We shall see!