Change user role based on total number of items ordered

You are using a wrong Parameter ‘$customer_id’ that is not defined in your code. Check below is updated code. It should be work. replace below code with your function.

function change_role_on_purchase( $order_id, $order ) {

    $user_id   = $order->get_customer_id();
    $user      = new WP_User( $user_id );
    $purchases = get_user_total_purchased_items( $user_id );

    if( $purchases >= 50 &&  $purchases < 100 && ! in_array( 'bronze_user', $user->roles ) ) {
        // Remove role
        $user->remove_role( 'subscriber' );

        // Add role
        $user->add_role( 'bronze_user' );
    }
    elseif( $purchases >= 100 &&  $purchases < 200 && ! in_array( 'silver_user', $user->roles ) ) {
        // Remove role
        $user->remove_role( 'bronze_user' );

        // Add role
        $user->add_role( 'silver_user' );
    }
    elseif( $purchases >= 200 && ! in_array( 'gold_user', $user->roles ) ) {
        // Remove role
        $user->remove_role( 'silver_user' );

        // Add role
        $user->add_role( 'gold_user' );
    }

}