Ligne 28 : clé inconnue. Tu essaye de lire une clé d'un tableau ($cases_cochees ?) qui n'est pas définie.

if(isset($_POST['jouer'])){
  if(isset($_POST['n'])){
    $cases_cochees = [];
}
    foreach( $_POST['n'] as $valeur){
      $cases_cochees[] = $valeur;
$nb=count($_POST['jouer']);
if($nb<=5) {
bla bla bla...

Fatal error: Uncaught TypeError: count(): Argument #1 ($value) must be of type Countable|array, string given in /home/clients/78f15c369c0786c4a419608e9e60028a/sites/xxxxr/lotoworld_tirage.php:24 Stack trace: #0 {main} thrown in /home/clients/78f15c369c0786c4a419608e9e60028a/sites/idealworld.fr/lotoworld_tirage.php on line 24

count() sur POST 'n', pas POST 'jouer'.

if(isset($_POST['jouer'])){
  if(isset($_POST['n'])){
    $cases_cochees = [];
}
    foreach( $_POST['n'] as $valeur){
      $cases_cochees[] = $valeur;
$nb=count($_POST['n']);
if($nb <= 5) {

Warning: Undefined array key 1 in /home/clients/78f15c369c0786c4a419608e9e60028a/ on line 28

Warning: Undefined array key 2 in /home/clients/78f15c369c0786c4a419608e9e60028a on line 28

Warning: Undefined array key 3 in /home/clients/78f15c369c0786c4a419608e9e60028a on line 28

Warning: Undefined array key 4 in /home/clients/78f15c369c0786c4a419608e9e60028a on line 28

Ya quoi à la ligne 28 de ton fichier /home/clients/78f15c369c0786c4a419608e9e60028a/... ?

if(isset($_POST['jouer'])){
  if(isset($_POST['n'])){
    $cases_cochees = [];
}
    foreach( $_POST['n'] as $valeur){
      $cases_cochees[] = $valeur;
$nb=count($_POST['n']);
if($nb <= 5) {
$sql="INSERT INTO JEUX (name, city, pays, continent, n1, n2, n3, n4, n5) VALUES ('$pseudo', '$citoyen', '$residant', '$situation_continent', '{$cases_cochees[0]}','{$cases_cochees[1]}','{$cases_cochees[2]}','{$cases_cochees[3]}','{$cases_cochees[4]}');";
$pdostat = $bdd->prepare($sql);
$pdostat->execute();

c'st la requete

j'ai remodifié le script ainsi croyant enfin atteindre mon but

if(isset($_POST['jouer'])){
  if(isset($_POST['n'])){
    $cases_cochees = [];
}
if (isset($_POST['jouer'] && count($_POST[n] >5))) {
echo "le nombre de cases à cocher est limité à 5";
}

    foreach( $_POST['n'] as $valeur){
      $cases_cochees[] = $valeur;

mais j'ai une autre erreur:
Fatal error: Cannot use isset() on the result of an expression (you can use "null !== expression" instead) in /home/clients/78f15c369c0786c4a419608e9e60028a/sites/idealworld.fr/lotoworld_tirage.php on line 22