老师，我的代码运行时老出现这个错误，这是为什么啊

我的代码运行时老出现这个错误ajax.js:59 GET http://localhost/ajax/server/isUserRepeat.php?username=15612345678 404 (Not Found)这是为什么啊

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
</head>
<link rel="stylesheet" type="text/css" href="css/css.css">
<body>
    
<div class="register">
  
  <div class="register_div_option">
   
   <span>登录</span>
   
   <span>注册</span>
  </div>
  
  <div class="form">
  
  <div class="register_div_tele">
   <span>+86</span>
   <input type="text" placeholder="请输入注册手机号" id="tele"/>
   <div id="check"></div>
   <div class="info" id="info"></div>
  </div>
  
  <div class="register_div_psw">
   <input type="text" placeholder="请设置密码" id="psw"/>
   <div id="pswcheck"></div>
  </div>
  
  <div class="register_div_btn">
   <input type="button" class="btn" value="注册"/>
  </div>
  </div>
</div>
<script type="text/javascript" src="js/ajax.js"></script>
<script type="text/javascript">
   var tele=document.getElementById("tele"),
      psw=document.getElementById("psw"),
      info=document.getElementById("info");
    check=document.getElementById("check");

      function checkTele(){
     var teleReg=/^1[3578]\d{9}$/;
     var teleValue=tele.value;
     if(!teleReg.test(teleValue)){
      info.innerHTML="手机号不合法";
      check.className="wrong"
     }
     else{
      info.innerHTML="";
      check.className="ok";
      //发起请求
      $.ajax({
       url:"http://localhost/ajax/server/isUserRepeat.php",
       method:"get",
       async:true,
       data:{username:teleValue},
       success:function(data){
        console.log(data);
       }
      })
      }
   }

      tele.addEventListener("blur",checkTele,false);


</script>
</body>
</html>

css.css

@CHARSET "UTF-8";
*{
padding:0;
margin:0;
}
html{
background-color:#505050;
opacity:0.9;
}
.register{
width:300px;
height:250px;
background-color:#E8E8E8;
margin:160px auto;
border-radius:2px;
}
.register_div_option{
padding:15px 20px;
}
.register_div_option span{
padding-right:10px;

}
.form div{
width:250px;
height:35px;
border:none;
margin:0 auto 25px;
border-radius:5px;
position:relative;
}
.form span{
background-color:white;
display:block;
width:33px;
height:35px;
float:left;
text-align:center;
line-height:33px;
}
.register_div_tele input{
float:left;
width:217px;
}
input{
width:100%;
height:100%;
border:none;
text-indent:5px;

}
input:focus{outline:0;}
input.btn{
border-radius:30px;
background-color:red;
}
div.register_div_tele div:first-of-type,div.register_div_psw div:first-of-type{
width:17px;
height:17px;
position:absolute;
right:10px;
top:9px;
}
div.register_div_tele div.ok,div.register_div_psw div.ok{
background:url(../img/icon.png) no-repeat 0 -67px;
}
div.register_div_tele div.wrong,div.register_div_psw div.wrong{
background:url(../img/icon.png) no-repeat -16px -67px;
}
.register_div_tele .info{
color:red;
position:absolute;
top:35px;
}

js.js

                   }
      }
     }else{
      throw new Error('No XHR object availabel.');
     }
     //xmlrequest对象状态变化的函数
     xhr.onreadystatechange=function(){
      if(xhr.readyState===4){
       if(xhr.status>=200&&xhr.status<300||xhr.status===304){
        callback&&callback(JSON.parse(xhr.responseText))
       }
       else{
        error&&error();
       }
      }
     }
     //创建发送请求
     xhr.open(method,url,async);
     //如果是post请求
     xhr.setRequestHeader("Content-type","application/x-www-form-urlencode");
     xhr.send(params);

  }
}