Why do we get the following error ‘No Stock’ while Posting Order ?

Cause :
The System will not allow you to post an order if the item is invalid or no stock is present of the items used in the Sales order. System will show messages ‘VTPOST06’, ‘Posting Unsuccessful’, ‘No Stock found for some item which is in scheme; or there is no stock to process any of the items of order’. Sometimes Unable to post the sale order,if stock is also available, but posting a log showing Expired item lot and short quantity for scheme item. Or Sometimes, There is found stock with near expiry date within expiry days range from the transaction date.
Remedies: Users should ensure that the lots selected in despatch does not come under near expiry. If the item validity permits, users can change in the Item master and Customer Item Master and re-enter the Minimum shelf life of the item to bring it back to use in Sales posting.

Why do we get the following error ‘Maybe Near Expiry or Expiry Product’ while posting