Stores still in "coming soon" mode, or locked behind a storefront password, block /products.json along with everything else on the site - there’s no way around that from outside the store, by design.

If you have the password yourself (it’s your own store, or a client’s pre-launch site), the workaround is the Chrome extension rather than this website: log into the store in a normal browser tab first, then click the extension’s icon on that same tab. Because it runs inside the page you’re already authenticated on, it can read the catalog the same way your browser can - the standalone website tool can’t hold that login state on your behalf.

Once the store goes fully public, either the website or the extension will work without needing the password at all.

One thing to check first, though: password protection and a simple 404 can look similar from a distance, so it’s worth confirming the store actually shows a password prompt in a normal browser tab before assuming that’s the cause - a mistyped URL produces a very similar empty result.