If in the past an item was out of stock and no longer available to sell, this attribute may have been set internally by yourself or a team member to update availability. This attribute may have also been changed due to an order cancelation. When an order is canceled with the Cancelation Reason applied was “Not Enough Inventory” this will auto-flag the item as “Not Sellable” stock but still should be flagged as sellable.
This can be corrected 1 of 2 ways. See Sellable Attribute Guide for a guide on how to update this attribute.