Can't click on Request Certificate for Let's Encrypt

Hello, I would like to install Let's Encrypt on one of my domains but I can't click on Request Certificate.

I have added SSL enabled site, saved it And when I go to Manage SSL Certificate, I can see that the current certificate is self-signed.

I go to Let's Encrypt tab, and I see this text : Let's Encrypt is a free, automated, and open certificate authority that can be used to generate an SSL certificate for use by Virtualmin. This page can be used to request a new certificate, which will overwrite any other you currently have configured for this domain. However, the Let's Encrypt service requires that your ownership of the certificate domain be validated by checking that this system hosts the website for the domain. This is done by placing a small temporary file in the website's document directory

I choose the domains of this server, Only renew manually and then try to click on Request Certificate. But nothing happens. Have I done something badly ?

Virtualmin 6.0 Centos 6.9

Thanks