Locate the code:
Add the following code below:
/**
* 2018-02-01
* How to fix the «Array to string conversion in Magento/Framework/DB/Adapter/Pdo/Mysql.php
* on line 2929» failure on a shipping address saving
* on the frontend checkout page in Magento 2.1.x?
* https://mage2.pro/t/5357
*/
if (is_array($s = $address->getData('street'))) {
$address->setData('street', implode("\n", $s));
}
if (is_array($s = $billingAddress->getData('street'))) {
$billingAddress->setData('street', implode("\n", $s));
}