Magento 2 – Fix Vendor Autoload Not Found Error

You could actually put it anywhere, what really matters is where you run it from. If you download composer and put it somewhere in your cli PATH then all you need to do is cd to your magento directory and then run "composer update", however you could also put it in the magento directory and run it from there.

You may already know but just to be clear. The CLI tool doesn't actually have anything to do with running the Magento application. The main thing Composer takes care of is downloading and installing the dependencies of the project, it will also create an autoload file so that those dependencies can easily be loaded from the application.

In order to install Github packages through composer.json you have to define it as repository like this:

{
    "repositories": [
        {      "url":"https://github.com/firegento/FireGento_FastSimpleImport2.git",
            "type": "git"
        }
    ],
}

if that doesn't work, then open your terminal and enter

composer config repositories.fastsimpleimport2 vcs https://github.com/firegento/FireGento_FastSimpleImport2.git

after that run the require command again.

composer require firegento/fastsimpleimport2