I am using Virtuemart version 3.4.2 on a Joomla 3.9 site. I find that the Checkout as Guest button has disappeard from the one of my sites. The relevant Checkout screen is here:
http://www.openphotographicsociety.org/shop-and-tools/shop/user/editaddresscartBT
Changing to another template makes no difference, but I have a very similar setup on Joomla 3.9, but a different template, and this works normally.
Embarassingly, I have no idea when it disappeared! The site has been through may updates and the Checkout as Guest button is rarely used - perhaps I now know why!!!
Any thoughts would be welcomed.
its your template
try
http://www.openphotographicsociety.org/shop-and-tools/shop/user/editaddresscartBT?template=protostar