https://wccftech.com/apple-pulls-watchos-5-1-following-reports-of-bricking-apple-watch-units/

Apple had seen fit to release its latest iOS 12.1, macOS 10.14.1, tvOS 12.1.1 and watchOS 5.1 to the general public yesterday. However, the company has pulled the watchOS 5.1 update, meaning you will temporarily not be able to the latest build if you haven’t already. Apple has done this for the good reason as some users have reported that their devices were bricked soon after installing the build. Let’s dive in to see some more details on the matter.

A small number of users have reported that their devices were rendered unusable by attempting to install watchOS 5.1. the Apple Watch gets stuck on the Apple logo upon startup. restarting the Apple Watch and the paired iPhone does not solve the issue. In this regard, Apple has now pulled the watchOS 5.1 update, so you will not be able to download it until further notice.