How to know why an SQL insert fails? (PHP/PDO)

Question

I am new to PHP so i am trying some very basic SQL queries. Basically the first SELECT query works fine and tell me if there's already a similar pseudo in the database, but then if there's isn't and I try to add it, it always returns Error and nothing is added into the database. Is there any way to have a more comprehensive error ? Thanks a lot if you can help

<?php 
$pseudo = 'Adrien';
try{

$db = new PDO(
    'mysql:host=localhost;dbname=tictactoe;charset=utf8',
    'root',
    'root');
    }
catch (Exception $e)
{
        die('Error : ' . $e->getMessage());
}
$sqlquery = $db->prepare('SELECT * FROM joueur WHERE pseudo= :pseudo');
$sqlquery->execute([ 'pseudo' => $pseudo ]);
$results = $sqlquery->fetchAll();

if(sizeof($results) >0 )
{
    echo 'This pseudo already exists.';
}
else
{
    $sqlinsert ='INSERT INTO joueur (pseudo) VALUES ( :pseudo)';
    $insertPseudo= $db->prepare($sqlinsert);
    if($insertPseudo->execute(['pseudo' => $pseudo]))
    {
        echo 'Success';
    }
    else
    {
        echo 'Error';
    }
    echo 'Welcome ' . $pseudo;
}

 ?>