请问一下我开了路由，为什么还报这个错误呢？ [ 2.0 版本 ]

An Error occurred while handling another error:
exception 'yii\base\InvalidRouteException' with message 'Unable to resolve the request "site/error".' in D:\root\PHPTutorial\WWW\an_git\vendor\yiisoft\yii2\base\Module.php:537
Stack trace:
#0 D:\root\PHPTutorial\WWW\an_git\vendor\yiisoft\yii2\web\ErrorHandler.php(108): yii\base\Module->runAction('site/error')
#1 D:\root\PHPTutorial\WWW\an_git\vendor\yiisoft\yii2\base\ErrorHandler.php(111): yii\web\ErrorHandler->renderException(Object(yii\web\NotFoundHttpException))
#2 [internal function]: yii\base\ErrorHandler->handleException(Object(yii\web\NotFoundHttpException))
#3 {main}
Previous exception:
exception 'yii\base\InvalidRouteException' with message 'Unable to resolve the request "".' in D:\root\PHPTutorial\WWW\n_git\vendor\yiisoft\yii2\base\Module.php:537
Stack trace:
#0 D:\root\PHPTutorial\WWW\an_git\vendor\yiisoft\yii2\web\Application.php(103): yii\base\Module->runAction('', Array)
#1 D:\root\PHPTutorial\WWW\an_git\vendor\yiisoft\yii2\base\Application.php(386): yii\web\Application->handleRequest(Object(yii\web\Request))
#2 D:\root\PHPTutorial\WWW\an_git\activity\web\index.php(17): yii\base\Application->run()
#3 {main}

Next exception 'yii\web\NotFoundHttpException' with message '页面未找到。' in D:\root\PHPTutorial\WWW\an_git\vendor\yiisoft\yii2\web\Application.php:115
Stack trace:
#0 D:\root\PHPTutorial\WWW\an_git\vendor\yiisoft\yii2\base\Application.php(386): yii\web\Application->handleRequest(Object(yii\web\Request))
#1 D:\root\PHPTutorial\WWW\an_git\activity\web\index.php(17): yii\base\Application->run()
#2 {main}