Magento 2 : getting Autoload error

If you take a look at vendor/magento/magento2-base/app/autoload.php, you'll notice following piece of code:

define('VENDOR_PATH', BP . '/app/etc/vendor_path.php');

if (!file_exists(VENDOR_PATH)) {
    throw new \Exception(
        'We can\'t read some files that are required to run the Magento application. '
         . 'This usually means file permissions are set incorrectly.'
    );
}

That means that either you don't have app/etc/vendor_path.php file, or you're not having enough permissions.

In order to fix permissions, you can run all of those commands, or at least the last two since you haven't mentioned that you've configured permissions on app/:

find . -type f -exec chmod 644 {} \;
find . -type d -exec chmod 755 {} \;
find ./var -type d -exec chmod 777 {} \;
find ./pub/media -type d -exec chmod 777 {} \;
find ./pub/static -type d -exec chmod 777 {} \;
chmod 777 ./app/etc
chmod 644 ./app/etc/*.xml