You can only create a Supplier Return if there is an original Purchase Order completed in Unleashed for the stock that is due to be returned. 

If you have stock on hand for a product that needs to be sent back to a Supplier, but the stock was not receipted against a Purchase Order in Unleashed, you can complete a Stock Adjustment to remove it from your inventory's stock on hand. 

For more details, see the following help file links:

• How to process a Supplier Return
• Add Stock Adjustments