Remove Postcode from Checkout
Beacuse the php validation return false, inside class Magento\Quote\Model\Quote\Address you will find validate function
First create the di.xml inisde you extension directly in folder etc YourVendor/YourExtName/etc/di.xml and fill :
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:...
That is magento's definition of one page checkout. Notice how the controller stays the same when switching between shipping and payment.
This terminology comes from Magento 1 where before 'One page checkout' the checkout was split into multiple steps and separate screens.
One page checkout was added after. If you are after truly on one page you will have ...