Apple is pushing out an update to stop temporary signing issues on the Watch.
They have been alerting developers about temporary signing issues that prevent apps from being installed onto the Apple Watch.
The full email reads:
We've discovered that there was a temporary issue with the App Store signature of your binaries that affects the ability of your app to install on Apple Watch. As a result, we're pushing an update with the proper signing to customers' devices.
Over the air updates are being pushed out now to affected developers only. It's not quite clear what went wrong on Apple's side.
This comes days before WatchOS 5 is due to be released to the public, with the event on Wednesday.