In XAMPP server, When you type the URL with https://localhost/ shows error 404 File or directory not found.

Solution,

We need to check the DocumentRoot directory in httpd-ssl.conf file.

#  cd /opt/lampp/etc/httpd.conf


<IfModule ssl_module>

# XAMPP

<IfDefine SSL>

Include etc/extra/httpd-ssl.conf

</IfDefine>

</IfModule>
Include etc/extra/httpd-ssl.conf

Open the file and update document root path,
 
#   vim /opt/lampp/etc/extra/httpd-ssl.conf


<VirtualHost _default_:443>

DocumentRoot "/opt/lampp/htdocs"

ServerName www.example.com:443

ServerAdmin you@example.com

ErrorLog /opt/lampp/logs/error_log

TransferLog /opt/lampp/logs/access_log
DocumentRoot  "/opt/lampp/htdocs"

Finally restart Xampp,
 
#  /opt/lampp/lampp restart