wordpress shortcode not working

Point-1: it’s not clear where you are getting $order_id from. I’m assuming it’s in the Shortcode.

Point-2: You don’t need to. use ob_* functions for returning Shortcode replacement content, simply returning is enough.

Assuming other parts of your CODE is correct, below is the possible CODE that should work:

1. If the shortcode is implemented in theme’s functions.php file:

   /**
    * Add Shortcode 
    */
   function tracking_shortcode( $attrs ) {
       $attrs = shortcode_atts( array(
           'order_id' => ''
       ), $attrs );
       if( empty( $attrs['order_id'] ) ) return '';
   
       $order = new WC_Order( $attrs['order_id'] );
       // I've followed your code, but may be even the above line is not needed.
       // may be you can just use $attrs['order_id'] directly to get tracking code
       return get_post_meta( $order->id, '_shipping_tracking_code', true );
   }
   add_shortcode( 'tracking_code', 'tracking_shortcode' );
   

2. If the shortcode is implemented using a plugin:

   /**
    * Add Shortcode 
    */
   function tracking_shortcode( $attrs ) {
       $attrs = shortcode_atts( array(
           'order_id' => ''
       ), $attrs );
       if( empty( $attrs['order_id'] ) ) return '';
   
       $order = new WC_Order( $attrs['order_id'] );
       return get_post_meta( $order->id, '_shipping_tracking_code', true );
   }
   
   /**
    * Initiate Shortcode 
    */
   function tracking_shortcode_init()
   {
       add_shortcode( 'tracking_code', 'tracking_shortcode' );
   }
   add_action('init', 'tracking_shortcode_init');
   

Learn more about shortcode from WordPress doc.