Multisite support not enabled

I’m copying a production site that IS working to a staging site that’s working but isn’t showing some graphical elements that ought to have come over in the WordPress importer process.

For the graphics that don’t appear on the staging site, when I try to open the graphic at its own URL, I get the message “Multisite support not enabled”. That’s it.

The wp-config.php files on Production and Staging both have the following set of lines to define multisite:

Production

$base = '/';
define('DOMAIN_CURRENT_SITE', 'www.example.com' );
define('PATH_CURRENT_SITE', '/' );
define('BLOGID_CURRENT_SITE', '1' );
define( "WP_USE_MULTIPLE_DB", false );

Staging

$base = '/';
define('DOMAIN_CURRENT_SITE', 'stage.example.com' );
define('PATH_CURRENT_SITE', '/' );
define('BLOGID_CURRENT_SITE', '1' );
define( "WP_USE_MULTIPLE_DB", false );

The one line that Production has, but that when put on Staging causes the “Error establishing database connection” message to appear, is this one:

define( 'SUBDOMAIN_INSTALL', false ); /** THIS WAS ADDED SO THAT EXAMPLE.COM WOULD REDIRECT TO WWW.EXAMPLE.COM INSTEAD OF GOING TO THE SIGNUP PAGE FOR A NEW SUBDOMAIN BLOG **/

.htaccess files on Production and Staging are identical.

On Staging there isn’t a Network Admin link in the dashboard for me to fiddle with its settings. That link does exist on Production.

I’m stuck. I don’t know how to a) enable Multisite on the Staging site and b) make the imported image appear and get rid of the Multisite error message.