After the server was down today (Sunday Oct 1st) my activity did not upload to the Garmin Connect App. However, I do have it saved on my garmin Vivoactive 4s watch. Can you advice me or tell me how can I move it into the app??
Open Garmin Connect, tap the sync icon.

Or do a manual upload to Garmin Connect Web - How Do I Manually Upload an Activity I Recorded to Garmin Connect?
You can also use Garmin Express if the app isn't working and you don't want to use Garmin Connect Web.
Could anybody advise how to sync
Garmin Connect: Sync Activities With Third-Party Websites
If you wish to manually transfer saved activities from a third-party fitness website into Garmin Connect, see - How Do I Manually Upload an Activity I Recorded to Garmin Connect?.