How to fix «Cannot read property 'code' of undefined» in `Magento_Checkout/js/view/progress-bar.js`?
The issue is fixed in Magento 2.2.8 and 2.3.1.
So to fix the issue in an ealier Magento version you can just replace the Magento_Checkout/js/view/progress-bar.js
script with the version from Magento 2.3.1: