Error on bncert-tool when entering the domain-name

Purpose & Context

I am trying to enable https for my lightsail instance, to do it I am following this "how to" document: https://lightsail.aws.amazon.com/ls/docs/en_us/articles/amazon-lightsail-enabling-https-on-wordpress

Problem

When bncert-tool ask me about the domain, I enter it and it shows me: Warning: The domain 'mydomain.com' does not resolve, please fix its DNS entries or remove it.

when I enter my domain in the navigator it works: it jumps into my wordpress instance.

Question

What am I missing? What should I do to fix it?

many thanks in advance