Why does a file from the root of a folder give a 404 error?

Question

Data storage

The essence of the matter is this. I need the js file to be in the root of the site and fired when requested from a third party server. In theory and according to the code from a third-party server, it should open in the browser on the request “domain/js”, but it is not available for me on this request, but is available on “domain//js” . What could be the problem and how can it be fixed?