This is a rare error that is usually resolved by restarting CostGuard.
However we have seen this error if you are in Receipts and are searching for an item number but you have NOT populated the screen with any inventory items.
Remember, in Receipts, right click and choose Fill in Transaction Values:
Or click on the plus button at the bottom left of the screen and choose the items you want.
Now you can search for item numbers.
Remember, if the screen does NOT have any items you will receive this error if you try to search for item codes.