Getting error: System.SecurityException: ViewAllData or ViewAllRecords required to access external id fields which do not have a unique index

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Getting error: DUPLICATE_VALUE, duplicate value found: readymade_inv__Inventory_Product_Code_Plus__c duplicates value on record with id

Please ensure that the Product Code used within the Salesforce Products is unique.

Please check you do not have duplicate Product Codes, i.e more than one Product using the same Product Code.