Hi Stevenzhu,
I flowed this tuto https://www.grafikart.fr/tutoriels/apache-ssl-letsencrypt-746.
The file .well-known is under the folder /opt/lampp/htdocs.
I try to added a file test.html under /opt/lampp/htdocs/.well-known/acme-challenge/ and it worked fine. Please helpe me to find the issue.
If you ran that command, it would have asked you to make some choices and perhaps enter some information. Could you please show us the complete output, including anything you typed in yourself?