缺少逗号结尾phone varchar(10) NOT NULL 第行，在将创建的表之后添加逗号。我已经测试过了https://prnt.sc/qgqmog

function lapizzeria_database() {
   global $wpdb;

   global $lapizzeria_db_version;
   $lapizzeria_db_version = "1.0";

   $table = $wpdb->prefix . \'reservation\';

   $charset_collate = $wpdb->get_charset_collate();

   //SQL Statement

   $sql = "CREATE TABLE $table (
        id mediumint(9) NOT NULL AUTO_INCREMENT,
        name varchar(50) NOT NULL,
        date datetime NOT NULL,
        email varchar(50) DEFAULT \'\' NOT NULL,
        phone varchar(10) NOT NULL,
        message longtext NOT NULL,
        PRIMARY KEY (id)
   ) $charset_collate; ";

   require_once(ABSPATH . \'wp-admin/includes/upgrade.php\');
   dbDelta($sql);
}
add_action(\'after_setup_theme\', \'lapizzeria_database\');