You have a typos here:

$stmt->bind_param(
"sisssssssssssssssssssssssssii", 
    $record["code"],
    $record["pairCode"],
    .....
    $record["variant:Dětská velikost"], // here is a mistake in the key. It should be "variant:Dětská Velikost"
    $record["variant:Množství"]
);

if (!$stmt->execute()) {
    echo "Error: " . $stmt->error . "<br>";
}

I reproduced your code as I could, corrected typos in the key and it works for me now. If it doesn't for you, please provide the full code.

Hope it helps. Cheers.

p.s. If you dump the $record var after binding it will be:

    array(2) {
  ["variant:Dětská Velikost"]=>
  string(30) "variant:Dětská velikost"
  ["variant:Dětská velikost"]=> // here is a broken / mistaken key after binding with null value
  &NULL

}