In Manager, File, Configuration, Options, Item options, enable "Display unknown item screen".
When an invalid entry is scanned or typed into Item Lookup Code, a window will pop up prompting the cashier to press Esc or Close before continuing.
If you experience unexpected results with this feature is enabled, make sure your scans do not contain spaces or tabs at the end of the barcode. To confirm valid scans, open Notepad in Windows and scan the item. The item lookup code or alias should list with the cursor resting on the next line. Hit the Backspace key to confirm the barcode ends with the last character of the code and not a space or tab.
If spaces appear your label format needs to be modified unless there really are spaces in your item lookup code (see Item Properties).
If a tab is inserted your barcode scanner needs to be reprogrammed.