Hello a can configure my apache webserver for letsencrypt certificate. But i can error.
My Command
sudo certbot certonly --email ****@gmail.com --standalone -w /var/lib/letsencrypt/ -d sth.dynu.net
And output
Saving debug log to /var/log/letsencrypt/letsencrypt.log
Plugins selected: Authenticator standalone, Installer None
Obtaining a new certificate
Performing the following challenges:
http-01 challenge for sth.dynu.net
Waiting for verification...
Cleaning up challenges
Failed authorization procedure. sth.dynu.net (http-01): urn:acme:error:connection :: The server could not connect to the client to verify the domain :: Fetching http://sth.dynu.net/.well-known/acme-challenge/KsuioyIhWBcdYzFcYL-uzGMu5f16A60wdd3zbbPj4-o: Timeout

IMPORTANT NOTES:
- The following errors were reported by the server:

Domain: sth.dynu.net
Type: connection
Detail: Fetching
http://sth.dynu.net/.well-known/acme-challenge/KsuioyIhWBcdYzFcYL-uzGMu5f16A60wdd3zbbPj4-o:
Timeout

To fix these errors, please make sure that your domain name was
entered correctly and the DNS A/AAAA record(s) for that domain
contain(s) the right IP address. Additionally, please check that
your computer has a publicly routable IP address and that no
firewalls are preventing the server from communicating with the
client. If you're using the webroot plugin, you should also verify
that you are serving files from the webroot path you provided.

Please tell me whats wrong ?

This file is to verify you are the owner of that domain and among few other things (auto renew), the certbot should generate one.

Helpful links:
https://tools.ietf.org/html/draft-ietf-acme-acme-03#section-7

https://certbot.eff.org/docs/using.html