can't use my retrieved variable after ajax Js PHP

Question

basically im trying to retrive a Js variable and send to php using ajax in order to add it to the data base ....and it works and it's added to the database but i can't use that variable in the php code (i need it in order to get the lastInsrtId() and i also need to echo it)

here is my JS code

let x;
function tableText(tableCell) {
     x =tableCell.innerHTML;
    console.log(x);
    $.ajax({  
    type: 'POST',
    url: 'reservation.php', 
    dataType: "json",
        data: { x:x },
    complete: function(text){
        console.log(text);
         return text; }
 });
}

function tableText(tableCell) {
     x =tableCell.innerHTML;
    console.log(x);
    $.ajax({  
    type: 'POST',
    url: 'try.php', 
    dataType: "json",
        data: { x:x },
    success : function(text){
            $('#MyDiv').html(text);
        }
 });
}

and then my PHP code

json_encode(['text'=>'ok']);
   $Salle=$_POST['x'];

echo $Salle;
   
 $bdd->beginTransaction();
$requete = $bdd->prepare('INSERT INTO reservation (code_salle,date_res,code_cr,code_profe)
 VALUE (:code_salle,:date_res,:code_cr,:code_profe)');
             $requete->execute(array(
                 'code_salle'=>$Salle,
                 'date_res'=>$date,
                 'code_cr'=>$heure,
                 'code_profe'=>$prof,   
             ));
  $last_id = $bdd->lastInsertId();
  echo "New record created successfully. Last inserted ID is: " .$last_id;


  $bdd->commit();

in the try.php page i get this error

Undefined index: x AND PDOStatement::execute(): SQLSTATE[23000]: Integrity constraint violation: 1048 Le champ 'code_salle' ne peut être vide (null)

but it's added to the data base i just can't use it in the php code(try.php page)

and also the console.log(text) in the JS works and the payload is x: stp0004 or whatver i clicked .....

in network preview there's no error and i can see the echo just fine but once in the page i get the errors