Answer: 

In order to make a product Never Diminishing, you cannot have any open transactions against it.

You will need to ensure that there are no parked sales orders, purchase orders, stock adjustments etc. 

There must also be no stock on hand for that product.

For more information, please refer to the Never Diminishing Products article.

Additional Information:

You can refer to the product's References and Transactions tab to review what transactions the product is involved in.

• To do this, navigate to Inventory | Products | View Products | Select your Product | Click onto either the References or Transactions tab.
• The Reference tab lists all incomplete transactions raised against that product.
• The Transaction tab lists all completed and/or closed transactions for that product.