Selection list value submited

Here is what I have for my php coding.

<?php
$USER_NAME = ' ';
$USER_PASSWORD = ' ';
$DB_SERVER = " ";
$DB_NAME = $USER_NAME."_db";

$conn = mysqli_connect($DB_SERVER, $USER_NAME, $USER_PASSWORD, $DB_NAME);
$result = mysqli_query($conn," SELECT * FROM employee,workson where workson.ESSN = employee.SSN and workson.Projnum ='" . $_GET["PNO"] . "'");
$notincluded = mysqli_query($conn, "SELECT SSN, FName, Lname FROM employee where NOT EXISTS (SELECT *  FROM workson WHERE employee.SSN = workson.ESSN and workson.Projnum ='" . $_GET["PNO"] . "')") ;


if (!$conn) {
    echo "Could not connect: ";
    echo mysqli_connect_error(); }

?>

<table border = "1">
<b>Employee List: <b>

    <td>Project Number</td>
    <td>SSN</td>
    <td>First Name</td>
    <td>Last Name</td>
    <td>Working Hours</td>
    <td>Delete</td>
    <td>Update</td>
</tr>

<?php
    $i=0;
    while($row = mysqli_fetch_array($result)) { 
    ?>
    <td style = "text-align:center; vertical-align: middle;"> <?php echo $row["Projnum"]; ?></td>
    <td style = "text-align:center; vertical-align: middle;"> <?php echo $row["SSN"]; ?></td>
    <td style = "text-align:center; vertical-align: middle;"> <?php echo $row["FName"]; ?></td>
    <td style = "text-align:center; vertical-align: middle;"> <?php echo $row["Lname"]; ?></td>
    <td style = "text-align:center; vertical-align: middle;"> <?php echo $row["NoHours"]; ?> </td>
    <td><a href="delete_employee.php?SSN=<?php echo $row["SSN"]; ?>&Projnum=<?php echo $row["Projnum"];?>">Delete</a></td>
    <td><a href="update_employee.php?SSN=<?php echo $row["SSN"]; ?>&Projnum=<?php echo $row["Projnum"];?>">Update Hours</a></td>
    </tr>
    
    <?php
    $i++; 
    } 
?>

</table>

<br>
<br>


<form method=POST action="http://mkimas.student.ust.hk/cgi-bin/mini_project_5.php">

<b> Employee not included : </b>
<?php
$notincluded = mysqli_query($conn, 
"SELECT SSN, FName, Lname FROM employee where NOT EXISTS (SELECT *  FROM workson WHERE employee.SSN = workson.ESSN and workson.Projnum ='" . $_GET["PNO"] . "')") ;
$employees = mysqli_fetch_all($notincluded);
$projnum = $_GET["PNO"];
print("<select name='Employees' id='employees'>");
foreach($employees as $employee){
   //You can use the index 0, which corresponds with the first selected column in query
   print("<option value='$employee[0]'>$employee[0]</option>");
}
print("</select>")
?>

<br>

Input the amount of hours and which project number new employee worked on: <br>
Hours: <input name=hours type =text> <br>
<input type=submit value="Submit">
</form>

For the second part of my php, a selection list of employees not included in the table above is printed. Then, I have the php file that has a SQL query that inserts the not included employee.

<?php
echo "Start \n";
echo "<hr>";

$USER_NAME = " ";
$USER_PASSWORD = " ";
$DB_SERVER = " ";
$DB_NAME = $USER_NAME."_db";

$ESSN = $_POST["Employees"];
$Projnum = $_POST["projnum"];
$NoHours = $_POST["hours"];

$conn = mysqli_connect($DB_SERVER, $USER_NAME, $USER_PASSWORD, $DB_NAME);

if (!$conn) {
    echo "Could not connect: ";
    echo mysqli_connect_error(); }
    
$sql="insert into workson(ESSN, Projnum, NoHours) values('$ESSN', '$Projnum', '$NoHours')";
$result=mysqli_query($conn,$sql);
 
if($result){
    echo "insert success";  } 
else{
    echo "insert fail";   }
 
mysqli_close($conn);

?>

Somehow, my coding always outputs "insert fail". What might be the problem?