Error When Installing Webmin Control Panel

I was doing tutorial How To Install Webmin on Ubuntu 20.04 https://www.digitalocean.com/community/tutorials/how-to-install-webmin-on-ubuntu-20-04

Everything was working until I got to the portion "Step 2 — Adding a Valid Certificate with Let’s Encrypt Webmin is already configured to use HTTPS, but it uses a self-signed, untrusted certificate. Let’s replace it with a valid certificate from Let’s Encrypt.

Navigate to https://your_domain:10000 in your web browser, replacing your_domain with the domain name pointing to your server’s IP address."

According to installation “Webmin install complete. You can now login to https://MarketingSales:10000/ as root with your root password, or as any user who can use sudo to run commands as root.”

Went to https://MarketingSales:10000/ and got "This site can’t be reached marketingsales’s server IP address could not be found. Try:

Checking the proxy, firewall, and DNS configuration Running Windows Network Diagnostics"

I tried using my already set up domains ang got “ERROR — DOCUMENT FOLLOWS This web server is running in SSL mode. Try the URL https://MarketingSales:10000/ instead.”

What needs to be done?