Exception #0 (Exception): Notice: Undefined index: id in /var/www/html/vendor/magento/module-config/Model/Config/Structure/Element/Iterator.php on line 63
One easy way to debug is: browse and open vendor/magento/module-config/Model/Config/Structure/Element/Iterator.php
change
public function setElements(array $elements, $scope)
{
$writer = new \Zend\Log\Writer\Stream(BP . '/var/log/config_debug.log');
$logger = new \Zend\Log\Logger();
$logger->addWriter($writer);
$this->_elements = $elements;
$this->_scope = $scope;
if (count($elements)) {
$lastElement = end($elements);
// Log last Element
$logger->info($lastElement);
$this->_lastId = $lastElement['id'];
}
}