Most often cause of this error is incorrect value passed for "StockItemId".

inventoryItem=
{"ItemNumber":"sample string 1",
"ItemTitle":"sample string 2",
"BarcodeNumber":"sample string 3",
"PurchasePrice":4.1,
"RetailPrice":1.1,
"Quantity":5,
"TaxRate":6.1,
"StockItemId":"ded577fa-eb0a-4986-b876-70d627b3dd8e"}

Please make sure that you have not used already existing in your system GUID for StockItemId and that there is no extra characters or bad formatting in this field.

You will need to generate random GUID by yourself for each newly created item.