Thanks for answer. I was able to make it work using DNS validation.
Honestly, I still do not understand how this method work, but is not important, the DNS solution makes so much more sense to me.
Now I have another similar issue.
It seem I have to use --csr command in order to have certbot verify a certificate I self signed.
I’m following this: Using certbot with --csr
This is what I get:
$ certbot certonly --csr unifi_certificate.csr.pem --staging --standalone
Saving debug log to /var/log/letsencrypt/letsencrypt.log
Performing the following challenges:
tls-sni-01 challenge for portal.cotech.co
Waiting for verification...
Cleaning up challenges
An unexpected error occurred:
The request message was malformed :: Error parsing certificate request: asn1: syntax error: sequence truncated
Please see the logfiles in /var/log/letsencrypt for more details.
Again, I face a similar issue: what is my server at portal.cotech.co supposed to answer and where?