Magento 2 core issues affecting checkout

Help! Sections titles show 100 and 200 instead of 1, 2

Ultimo theme compatibility

Get customer comments to order confirmation e-mail

Make telephone field optional

Terms and conditions checkbox

How to add header and footer to checkout page

Placing an order with quote_id xxxx is failed

Getting error on order placement time: Placing an order with quote_id xxxx is failed: No such entity with cartId = xxxx