Discounts can only be applied for a quantity of two or more for a guest customer. If you have the Standard plan or higher, you can offer a discount for a quantity of one for a tagged and logged in customer.
As a workaround, if you would like to offer a discount to a default customer for a quantity of one, you can simply change the product price in your Shopify admin.
If you do not wish to show the first row of the quantity breaks grid at all, you can remove it by navigating to the Quantity Break Display settings and disabling the Display base price and quantity setting.