I am working on my websites login system. But when I login it goes through all my security checks and then when it is suppose to start the session it gives me this error.

"Cannot send session cookie - headers already sent by (output started at /Applications/XAMPP/xamppfiles/htdocs/Site/Sign_In.php:7)"

Here is my code
PHP Code:
               <?php

            session_start
();
        
        include 
'Include/Database.php';
        
        
$Mail mysql_real_escape_string(strip_tags(trim($_POST['Mail'])));
        
$Password mysql_real_escape_string(md5(strip_tags(trim($_POST['Password']))));
    
        if (
$Mail && $Password) {
    
            
$Query =  mysql_query("SELECT * FROM Users WHERE Email='$Mail' AND Enabled='1'");
            
            
$Rows_Number mysql_num_rows($Query);
            
            if(
$Rows_Number==0) {
            
                echo 
'<span class="Label" style="color: #CC0000;">Username or password is incorrect</span>';
            
            }else{
            
                while(
$Fetch_Row mysql_fetch_assoc($Query)) 
                {
                
$DB_Mail $Fetch_Row['Email'];
                
$DB_Password $Fetch_Row['Password'];
                }
                
                if(
$DB_Mail==$Mail && $DB_Password==$Password) {
                    
                    
$_SESSION['User_Session'] = $DB_Mail;
                    echo 
$_SESSION['User_Session'];
                
                }else{
                
                    echo 
'<span class="Label" style="color: #CC0000;">Username or password is incorrect</span>';
                
                }
                
            }
            
        }else{
        
            echo 
'<span class="Label" style="color: #CC0000;">Your must enter you mail and password</span>';
            
        }
                
?>
Does anybody know the problem?