Why are we unable to post order, when system shows error ‘Posting Unsuccessful-Available Quantity in stock is less than quantity to be allocated’?

Cause:
System shows the error ‘Posting Unsuccessful-Available Quantity in stock is less than quantity to be allocated’ when the item used in Sales order has quantity insufficient for the despatch.
Suggestion
user should ensure the stock is sufficient of the item used in Sales order.Lot and lot sl number are properly selected of the available stock.