That error would typically mean that another service is already listening on the port you want (443). Do you have a web server or another service listening on this port? Is there maybe another coturn process already running?
You can use ss -l4 to see what ports your services are listening to in the IPv4 space, and you can use ps -A|grep coturn to look specifically for other coturn processes.
Another possibility is that the user account that’s running coturn doesn’t have permission to run on low-numbered ports. Did you follow the instructions below “Anpassung des systemd für die Bindung von Coturn auf eine Port < 1024.” in the article?
If your server is running behind NAT, it won’t be able to listen on the WAN IP. In that case, the config section for AWS (and other NAT) users might be helpful. There’s also this how-to on the forums, which you can compare against your article to see what’s different.