How can I install a SSL certificate on Ubuntu server that does not have a GUI interface?
You would first need to generate the CSR for your domain on the server.
You can now order the SSL certificate from your provider using the CSR file you just generated, server.csr.
Copy the Certificate files to your server. Download your Intermediate (DigiCertCA.crt) and Primary Certificate (your_domain_name.crt) files from your Customer Area, then copy them to the directory on your server where you will keep your certificate and key files /etc/ssl/certs/. Make them readable by root only.
All you need to do from here now is to redirect all traffic from HTTP to HTTPS in the .htaccess file. You can just add a redirect rule.