To force at least one value to be selected for a product variant on the POS, tick Force Selection on the backend (Products > Product Variants > Edit > Force Selection).
On the POS, the first value listed will be selected by default.
E.g. a drink size must be defined as small, medium, or large- if the staff member does not specify what size the customer wants, in this case it will be a “small”. Without “force selection”, the staff would be able to put through drink order without mentioning the size.