Alter users table

Hello,

I have create plugin that if we activated it, it will add new column on users table.

the code :

global $PWTW_db_version;
$PWTW_db_version = '1.0';

function PWTW_install() {
	global $wpdb;
  global $PWTW_db_version;

	$table_name = $wpdb->prefix . 'users';

	$sql = "ALTER TABLE $table_name ADD COLUMN <code>user_credits</code>  int(20) NOT NULL AFTER <code>display_name</code>;";

	require_once( ABSPATH . 'wp-admin/includes/upgrade.php' );
	dbDelta( $sql );

	add_option( 'PWTW_db_version', $PWTW_db_version );
}

$installed_ver = get_option( "PWTW_db_version" );

if ( $installed_ver != $PWTW_db_version ) {

	$table_name = $wpdb->prefix . 'users';

	$sql = "ALTER TABLE $table_name ADD COLUMN <code>user_credits</code>  int(20) NOT NULL AFTER <code>display_name</code>;";

	require_once( ABSPATH . 'wp-admin/includes/upgrade.php' );
	dbDelta( $sql );

	update_option( "PWTW_db_version", $PWTW_db_version );
}

function PWTW_update_db_check() {
  global $PWTW_db_version;
    if ( get_site_option( 'PWTW_db_version' ) != $PWTW_db_version ) {
        PWTW_install();
    }
}

register_activation_hook( __FILE__, 'PWTW_install' );
add_action( 'plugins_loaded', 'PWTW_update_db_check' );

i try to Active the plugin, but it not create new column in users table.

does anyone can light me up?

thank you