FAQ: One Step Checkout For Magento 2
The thing is that VAT field needs to be enabled on Magento first. Having done it, please proceed to a full guide how to make VAT field visible on the checkout page.
Address Auto Suggest allows to fill in shipping/billing address fields using only a part of address as a base. Read the full answer to get an explanation on how it works and why it is handy.
This guideline covers whole sequence of needed actions, where possible. Proceed to the full answer to get a step-by-step guide.
Most common difficulities (and most effective solutions for these) related to Address Auto Suggest are described here. Proceed to the full answer to get a step-by-step guide.