Redirect to the cart instead order place or Yaml class is not available
Problem
On the last step checkout when you click the "Place order" button with a redirect to your cart happens order place instead.
Use Case
The most possible reason is: some of the necessary packages are not installed or have the wrong version.
Solution
You should backup all your store files before the packages upgrade!
composer upgrade
You should upgrade composer packages installed on your server, you should exec the following command in the Magento folder: