Force stop & restart the app. And repeat the sync couple of times. Vivoki is not easy to sync with recent versions of Garmin Connect, because it is a discontinued 10 years old device, so the new versions of the app are not really tested with it. If you were on Android, I'd advise to install an old version of the GC app, but on iOS you do not have much choice. Still, with some patience, Vivoki does sync after couple of attempts. And if it does not, then you may need to change the battery.
