If you search for a product in your POS, it may come up with a quantity of "N/A" and say "No Variants" under the price. While normally out of stock products can be added to the POS and go into negative quantity, products that show up like this cannot.
The reason for this is these products have never had qty or opt qty adjusted previously. When an item is newly imported or created, none of its conditions have any numerical value. So, instead of having 0 in stock, they have nil in stock. The condition variant isn't created for that item until the first time a change is saved to it. This is why it will show up as "No Variants" in the POS.
For an individual product, the way to fix this is to search for it in your admin on the Inventory page. click "Manage" and adjust either the qty or opt qty and save. Now, it can be selected in your POS.
If you are wanting to convert a whole category from nil values to 0 to make them selectable, there is a batch update you can do. What you'll want to do is select the category and product type on your inventory page. Check the box for the condition you want to convert to a real 0. Under the batch update options, select "increment opt_qty by" and enter "0" into the field.
When you batch update, this will increment the opt qty for the products in the category by 0, essentially making no changes to them, but still converting the nil quantities into 0s.
IMPORTANT: When doing this with Magic Singles, you should not select all Languages. Only batch update your English Magic Singles in this manner. If you include multiple languages, it will create too many variants on your database. This will cause batch updates to take much longer, and could affect the performance of the POS when searching up products.