JavaScript validation error. What’s the matter? – Blancer.com Tutorials and projects

<script type="text/javascript">
    function checkF()
    {
        var f = document.user_info;
        
        if(f.userID.value.length < 2 || f.userID.value.length >16)
            {
            alter("아이디는 2~16자 이내로 입력해야 합니다.");
            f.userID.focus();
            return false;
            }
        else if(f.userPW.value.length < 6)
            {
            alter("비밀번호는 6자 이상으로 입력해야 합니다.");
            f.userPW.focus();
            return false;
            }
        else if(f.userMAIL.value == "")
            {
            alter("이메일을 입력해야 합니다.");
            f.userMAIL.focus();
            return false;
            }
        else return true;
    }
</script>
</head>
<body>
    Home > 회원 가입
    <hr>
    <form action="insertDB.jsp" name="user_info" method="post" onsubmit="return checkF()">
     <fieldset style="width:230px">
      <legend> 회원 가입 화면 </legend><p>
            
            아이디 : <br>
            <input type="text" size = "16" name="userID"><br><br>
            
            비밀번호 : <br>
            <input type="password" size = "16" name="userPW"><br><br>
            
            이메일 : <br>
            <input type="email" size = "30" name="userMAIL"><br><br>
            <hr>
            <input type="reset" value=" ◀ 다시작성 ">
            <input type="submit" value=" 가입하기 ▶ ">
            <hr><hr>
     </fieldset>
    </form>
</body>
</html>

An error was found in the validation step that prevented validation from being performed properly.
onsubmit=”return false” works fine. Perhaps it is a connection problem. Why can’t I connect?

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<%@ page import="java.sql.*" %>
<%@ include file = "dbConn.jsp" %>
<%
    String u_id = request.getParameter("userID");
    String u_pw = request.getParameter("userPW");
    String u_mail = request.getParameter("userMAIL");
    
    String sql="INSERT INTO members(id, passwd, email) VALUES";
    sql += "('" + u_id + "','" + u_pw + "','" + u_mail +"')";
    
    Statement sm = conn.createStatement();
    
    int count = sm.executeUpdate(sql);
    if(count ==1){
        response.sendRedirect("signupSuccess.jsp");
    }else{
        out.println("회원가입 실패!");
        response.sendRedirect("signup.jsp");
    }
    sm.close();
    conn.close();
%>

This is the contents of insertDB. I’m attaching it in case there might be a cause here.

If there is no problem with the code, is the external factor the problem? I am wondering what steps to take to solve this.