If a Product has a Prepack applied, it will force you to enter quantities using the special Prepack cell.
To override this Prepack constraint on the Seller side, there are 2 options.
Disable Prepacks for entire Order
In the upper right corner of an Order, click "More" and select "Hide Prepacks".
All products with Prepacks will have their Prepack QTY cells hidden and allow you to enter any value into the size QTY cells.
Override Prepack for individual Product
To override the prepack restriction on a single Product, simply click on the Prepack cell label and the ordering will "flip" to open ordering in the Size cells.