https://www.cnet.com/news/apple-pulls-watch-os-5-1-update-after-problems-caused-for-some-users/

Due to a small number of Apple Watch customers experiencing an issue while installing watchOS 5.1 today, we've pulled back the software update as a precaution. Any customers impacted should contact AppleCare, but no action is required if the update installed successfully. We are working on a fix for an upcoming software update.

News of the problems with the software update was previously reported by 9to5mac.