Why can't I see my newly created modifier on the POS screen?

Why can't I see my newly created modifier on the POS screen?

This is so because modifiers are attached to the Menu Items and would therefore appear only when a Menu Item is ‘modified’ on customer’s request. 
Plugins in Tagrain are either Subscription Based or License Based. So the price that you pay for your plugins is based on the number of POS terminals that you have in your setup and the number of licences that you have acquired. There will be no change in your subscription amount in the current billing cycle if you are only deactivating the plugin AND not updating your Store / POS count.
You don't need to subscribe to the Hospitality for Quick Serve and Fine Dining plugin, as the plugin is already included in both the editions - Community and Enterprise. This means that you will NOT be required to purchase and subscribe to this plugin. You just need to activate the plugin from the Purchase Subscription Screen to unlock the Hospitality features.
Activating or Deactivating the Hospitality for Quick Serve and Fine Dining plugin will not affect your subscription amount for your next billing cycle because the plugin is free.