Thanks for ping me.
I remember that duckdns can only create one txt record for a sub domain.
So, there is a trick if you need to create wildcard certs for your domain.
please issue a normal cert for the root domain first.
acme.sh --issue -d rootdomain.com --dns dns_duck ......
Once it successes, try to issue a wildcard domain:
acme.sh --issue -d rootdomain.com -d '*.rootdomain.com' --dns dns_duck .....
It should work.
Thanks