Thanks. We will try that reversal. But, as noted above the command works perfectly fine on other Gatsby installations. We also just tested a new gatsby installation and command worked fine. It is just throwing an error for this one installation.
As for the files, the idea is that Netlify doesn’t know what the file netlify_headers is, so you most certainly need to copy it and rename it to headers, otherwise it won’t know what to do with the file. And the idea is not to call it _headers originally so it is only processed for the branch-deploy. We simply followed the instructions in this post: Password protection for subdomains and specific pages - #6 by Vladimir