How to use PageSpeed in a test environment? (password protected)

Google's PageSpeed do not run in a test environment of a website (ProdPreview). I guess it's due to the password protection in it. Is it possible to make PageSpeed run in this environment? If yes, how to?

PageSpeed give me back the message below, when I try to run it in the ProdPreview environment:

"Lighthouse returned error: ERRORED_DOCUMENT_REQUEST. Lighthouse did not reliably load the requested page. Make sure you are testing the correct URL and that the server is responding appropriately to all requests. (Status Code: 401) Dispensing"

No code at all. It's just a public tool being used in a private environment.

Expected result: Get the PageSpeed analysis result. Actual result: As described in "#2 = Provide background including what you've already tried"