If the HTTP and HTTPS ports (80 and 443, respectively) are privileged ports on your OS, you will either need to run with elevated privileges or use a higher port. To use a higher port, just change the address to something like <code>localhost:2015</code> and change the HTTP port using the <ahref="/docs/caddyfile/options">http_port</a> Caddyfile option.
You will probably be asked for your password, because Caddy serves all sites -- even local ones -- over HTTPS by default. (The password prompt should only happen the first time!)
For local HTTPS, Caddy automatically generates certificates and unique private keys for you. The root certificate is added to your system's trust store, which is why the password prompt is necessary. It allows you to develop locally over HTTPS without certificate errors.