Assuming certbot is in the path then
certbot certonly --standalone -d pleurx-demo.verisign.tech
if not then you might need to go to the directory where you installed certbot and use
./certbot certonly --standalone -d pleurx-demo.verisign.tech
Assuming certbot is in the path then
certbot certonly --standalone -d pleurx-demo.verisign.tech
if not then you might need to go to the directory where you installed certbot and use
./certbot certonly --standalone -d pleurx-demo.verisign.tech