SSL error: DNS record exists for this domain

I created amazon lightsail instance and trying to make it secure with lets encrypt ssl.

I get following error when i run this command

/home/bitnami/letsencrypt/letsencrypt-auto certonly -w /home/bitnami/apps/wordpress/htdocs -d yourdomain.com.au -d www.yourdomain.com.au

how to fix ?