I have installed the GA-4 Analytics update to Zen Cart 1.5.6c however I get an error in the log and Web site doesn't load

PHP Parse error: syntax error, unexpected '?' in /includes/classes/observers/class.ga4_analytics.php on line 309

case FILENAME_SPECIALS:
global $specials, $listing;

$products = $listing ?? $specials ?? []; <<line 309
if (empty($products)) {
break;
}
$_SESSION['ga4_analytics'][] = [
'event' => 'view_item_list',
'parameters' => [
'item_list_name' => GA4_ANALYTICS_SPECIALS,
'items' => $this->getListingItems($products),
]
];
break;


I have put the G- code in the GA4 Analytic admin field.

Any ideas?