- Choose "Required" or "Optional" in the Show Tax/VAT Number drop-down menu in Stores > Customers > Customer Configuration > Name and Address Options.
- Scroll up to "Create New Account Options" and enable "Show VAT Number on Storefront".
- Finally, go to System > One Step Checkout > Manage Checkout Fields drag and drop VAT filed from disabled fields section to Enabled. Save changes and flush Magento caches.
Check the website frontend. VAT field is now shown on the checkout page.
→ Create limitless custom checkout fields with the help of the Order Attributes plugin