The full in-case it help :
<!doctype html>
<?php include ("DBConect.php");?>
<html>
<head>
<style>
.red {
color:rgba(221,26,29,1.00);
font-weight:bold;
}
h1 {color:rgba(237,17,21,1.00);
align-items:center;
}
</style>
<meta charset="utf-8">
<title>Insert Data</title>
</head>
<body>
<?php
echo "<h1>Add Units to owner:</h1>";
?>
<form method="post" action="">
<table>
<tr>
<td colspan="3"><strong>Insert Owner Information down:</strong></td>
</tr>
<tr>
<td width="200"><b>Select Owner</b><br></td>
<td width="6">:</td>
<td width="100"><select name="name">
<?php
$sql = mysqli_query($conn ,"SELECT name FROM owners");
while ($row = mysqli_fetch_array($sql)){
?>
<option value="name"><?php echo $row['name']; ?></option>
<?php
// close while loop
}
?></td>
</tr>
<tr>
<td width="200"><strong>Select Property</strong></td>
<td width="6">:</td>
<td width="100"><select name="pr_num">
<?php
$sql5 = mysqli_query($conn ,"SELECT PR_num FROM property");
while ($row = mysqli_fetch_array($sql5)){
?>
<option value="PR_num"><?php echo $row['PR_num']; ?></option>
<?php
// close while loop
}
?></td>
</tr>
<tr>
<td colspan="3"><input name="submit" type="submit" id="submit" value="Submit"></td>
</tr>
</table>
</form>
<br>
<br>
<?php
if (isset($_POST['submit'])) {
/*$_If_Exist = mysqli_query($conn, "select PR_num from property where PR_num = '$_POST[PR_num]'");
if (mysqli_num_rows($_If_Exist) > 0) {
die("<p class='red'>Sorry Unit # allready Exeist</p>");
}
*/
//Here the SQL Command to insert data in Property table
$sql2 = "SELECT O_ID FROM owners WHERE name = '$_POST[name]'";
$result1 = mysqli_query($conn , $sql2);
if ($result1==false) {
die("failsd".$conn->error);
}
$row = $result1->fetch_object();
$own = $row->O_ID;
$sql3 = "SELECT Pr_ID FROM property WHERE PR_num = $_POST[pr_num]";
$result2 = mysqli_query($conn , $sql3);
$row = $result2->fetch_object();
$pro = $row->Pr_ID;
$sql4= "INSERT INTO owner_property (Owner_ID, Pr_ID) VALUES ($own, $pro)";
if (!mysqli_query($conn, $sql4)) {
die("Faild:". mysqli_error($conn));
}
echo "<p class='red'>Unit added to $_POST[name]</p>";
unset($_POST);
}
mysqli_close($conn);
?>
</body>
</html>