Please share your Caddyfile. Can’t help without seeing what you actually have in your config.
That error typically means you’re listening on a unix socket or something, therefore Go’s stdlib can’t automatically determine a port to use for the Alt-Svc header to announce that HTTP/3 support is available to clients. It’s not really a problem, it just means that the Alt-Svc header won’t be used so clients won’t try HTTP/3.