Julie here from Shopify Support.
In what situations are you wanting to cancel an order? Typically, an order can only be cancelled if you have not collected payment for the order, or if you have collected payment but none of the items have been fulfilled.
In your POS app, are your fulfillment settings set to automatically mark orders as fulfilled or unfulfilled? You can confirm this in the POS app by heading to Store > Settings; by default, all POS orders are marked as fulfilled unless otherwise specified within these settings. Because of these conditions, cancelling orders wouldn't necessarily apply on POS orders, as payment would have been captured at the time of purchase and items are generally automatically fulfilled.
When cancelling orders, are you also wanting to refund them? If that's the case, you can refund orders directly through the POS app. Once an order is refunded, it will be marked as such in the Orders page in both the POS app and your Shopify admin. This essentially indicates that the order is "cancelled". Once you refund the order, you could also add a cart note to the order, indicating why the order was refunded/cancelled.
I hope this clears things up and I'd love to hear more about your use case for cancelling POS orders. Feel free to respond here and let me know!