How to properly install nesk/puphpeteer (Ubuntu, PHP)?

Question

PHP

The instructions are simple

composer require nesk/puphpeteer
npm install @nesk/puphpeteer

but after using simple example

require('vendor/autoload.php');
use Nesk\Puphpeteer\Puppeteer;
$puppeteer = new Puppeteer;
$browser = $puppeteer->launch();

Gives me an error

Parse error: syntax error, unexpected '?' in /var/www/html/vendor/nesk/puppeteer/src/Puppeteer.php on line 60

See what's in that line

private function currentPuppeteerVersion(string $nodePath): ?string {

Tell me what should I do in this case?
PHP 7.2, Ubuntu 0.16.04.5