Error Occurred

<h1>Error Occurred</h1><p>syntax error, unexpected token "??", expecting variable</p><p>In /var/www/systemdev.karlkratz.de/src/Core/Config/PathResolver.php on line 18</p><h2>Stack Trace:</h2><pre>[
    {
        "file": "/var/www/systemdev.karlkratz.de/src/Helpers/ConfigLoader.php",
        "line": 34,
        "function": "{closure:/var/www/systemdev.karlkratz.de/src/bootstrap.php:5}"
    },
    {
        "file": "/var/www/systemdev.karlkratz.de/src/Helpers/ConfigLoader.php",
        "line": 43,
        "function": "__construct",
        "class": "SystemAdmin\\Helpers\\ConfigLoader",
        "type": "->"
    },
    {
        "file": "/var/www/systemdev.karlkratz.de/src/Services/SecurityHeaderService.php",
        "line": 18,
        "function": "getInstance",
        "class": "SystemAdmin\\Helpers\\ConfigLoader",
        "type": "::"
    },
    {
        "file": "/var/www/systemdev.karlkratz.de/public/index.php",
        "line": 33,
        "function": "__construct",
        "class": "App\\Services\\SecurityHeaderService",
        "type": "->"
    }
]</pre>