To install a free LetsEncrypt SSL certificate on bitnami WordPress installation, do the following

log in to the server as user “bitnami” using SSH/putty.

Run command

sudo /opt/bitnami/bncert-tool

It will ask for the domain name. Enter domain names separated by space. After that you will be asked if you need to redirect the domain name to www, select the one you prefer for your website. Then it show a summary of tasks, once you confirm it, SSL will be installed.

Before you do this, make sure the domain name is pointed to server IP and DNS is propagated.