NODE_TLS_REJECT_UNAUTHORIZED

[efeakaroz13]

Output:Warning: Setting the NODE_TLS_REJECT_UNAUTHORIZED environment variable to '0' makes TLS connections and HTTPS requests insecure by disabling certificate verification.

[arseniov]

You can ignore that, as long as it's used in a prod environment.
Try adding --verbose to your options in the command line and you should see that the process starts correctly.

sitemap-generator --verbose https://example.com