I am using artifactory and have no issues with the npm solution. However, I have set up composer using the "Set Me Up" instructions, i.e. installed the ~/.composer/config.json & ~/.composer/auth.json files. I now get the following error when I try to perform a composer install:
$ dcomp install
Loading composer repositories with package information
[Composer\Downloader\TransportException]
The "https://myartifactory.jfrog.io/artifactory/api/composer/php-local/packages.json" file could not be downloaded (HTTP/1.1 404 Not Found)
install [--prefer-source] [--prefer-dist] ...
The user I have configured in auth.json is an admin user, and I've pushed a single package into the Artifactory. Additionally, I noticed the url contains plural packages.json as opposed to singular package.json, so I am assuming this is an issue with the artifactory configuration itself and not the package I've uploaded.
The issue in my case was as a couple commenters observed: the index wasn't being generated.
For me, the fix was a matter of using a .zip compressed file to upload composer packages instead of a .tar.gz. I can't tell you why a tarball won't work, especially considering Artifactory's documentation explicitly mentions tar.gz, but it would appear .zip is the way to go.