Error when using php ajax to check input data with the system

I am learning how to use php ajax to check if registered email data already exists in the system. But when used, the result returned from success:function(data) is always the source code of the original php page (as shown in the image below).
Here is my javascript code

            URL: "process-email.php",
            type: "post",
            data: {email:$(this).val()},
            success:function(res) {

This is the destination for data processing

    include "config/config.php";
    $result = mysqli_query($conn,"SELECT * FROM tb_user WHERE email="" . $_POST["email'] . "'");
    if(mysqli_num_rows($result) <= 0)
        echo "OK.";
        echo "Already exist.";

And here is the data sent to my main site

Is there any way to fix it? Thanks everyone!


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