Hello, I cannot start a workout, the app tells me that the synchronization is not finished. Yet I did it 100x on the app and on the watch ... Any solutions? Thanks!

Try syncing over the USB cable and Garmin Express, to see whether it makes any difference, or whether it shows an error message revealing the reason better.

Also try rescheduling the workout, or making a minor change in it, and then sending to the device again.