Sync Issues since Update to Version 5.00

I've seen this issue on other forums. Try this: Remove the watch from Garmin Connect, "Forget" the device in your Bluetooth settings on your phone. Restart your phone. Add the watch back to Garmin Connect.

I tried to reconnect the watch three times, but it didn’t solve the problem.

Sometimes just power-cycling your device and phone and/ syncing with Garmin Express on your PC or Mac will fix it. Otherwise you can try the more extensive list of troubleshooting steps here: A more extensive list of things to try here: Sync / Connection Troubleshooting Guide  It's on the Android forum but most of it also applies to iOS

Ultimately, if still no joy, you'll need to contact Garmin Support. https://support.garmin.com

before you do anything, check if Garmin Connect service are up and running, the last days there was a prob with the availibility of the servers, check here:

https://connect.garmin.com/status/

I have the same issue, after the hack humiliation and now not being able to sync after multiple restarts and a fresh install of Garmin Connect I'm slowly coming round to thinking it might be time to set what the competition has.

I'm on holiday with no laptop with Garmin Express so this is doubly frustrating. 

You may want to try following what seilogramp suggested but make sure that your phone BT does not link with the watch, it should be the Garmin Connect Mobile app which sets up the connection.

This is the solution I used over the last few years with same problems.