Error 'Can not modify header information' I have already deleted the blanks

Good morning,

I have deleted spaces between the tags ?> and <?php because this had already happened before, but this time, even though I'm looking for no response, I leave the error and the complete code in which I want to redirect panel_de_control.php

I get the error:

  

Warning: Can not modify header information - headers   already sent by (output started at   /home2/gabriel/contabilidad.dsinsufer.com/PHP/conectar.php:10) in   /home2/gabriel/contabilidad.dsinsufer.com/PHP/checklogin.php on line   18

<?php
session_start();
include('../PHP/conectar.php');
$username = $_POST['username'];
$password = $_POST['password'];
$sql = "SELECT * FROM 'usuarios' WHERE 'usuario'='$username';";
$result = $conexion->query($sql);
if ($result->num_rows > 0)
{
    $row = $result->fetch_array(MYSQLI_ASSOC);
}
if (password_verify($password, $row['contrasena']))
{
    //AQUI GUARDO LAS VARIABLES DE LA BD PARA SER USADAS EN PANEL-CONTROL.PHP
  $_SESSION['loggedin'] = true;
    $_SESSION['usuario'] = $username;
    $_SESSION['start'] = time();
    $_SESSION['expire'] = $_SESSION['start'] + (5 * 800000);
    //redirigir automaticamente
    header('Location: panel_de_control.php');
    //echo "Bienvenido(a)! ".$_SESSION['usuario'];
    //echo "<br><br><a href=panel_de_control.php>PANEL DE CONTROL</a>";
}
else
{
  echo "Usuario o contraseña estan incorrectos.";
    echo "<br><a href='../login.html'>Volver a Intentarlo</a>";
}
 mysqli_close($conexion);
?>

Here the connect.php

<?php
$conexion = mysqli_connect('localhost', '*****', '*****','bd_contufer');
if (mysqli_connect_errno())
  {
    echo "No se pudo conectar, mostrando error de MySql: " . mysqli_connect_error();
  }
  else
  {
    echo 'Conexion exitosa';
  }
?>

Any ideas?