[SOLVED] Error activating Let's Encrypt

I’m following the step in https://www.digitalocean.com/community/tutorials/how-to-install-webmin-on-ubuntu-16-04, but when i request a new certificate, it occurs the following error:

Requesting a new certificate for my.domain, using the website directory /var/www/html …
… request failed : Failed to request certificate :

my.domain challenge did not pass: Invalid response from http://my.domain/.well-known/acme-challenge/CCZqnto74VD0TQAekghFEbDzdr5talFXxq2itkCro5k:

404 Not Found

Not Found


What i’m doing wrong?

I formatted the server and the service started up again.