after you connect your barcode reader go to Configuration > Point of Sale > Connected devices and add your barcode reader under IOT Box if the brand needs it or directly choose connect device if it doesn't require IOT Box

Odoo POS will work with most USB Barcode Scanners, connected directly on your computer. Bluetooth barcode scanners and network receipt printers are supported natively. Of all compatible barcode scanners, Odoo recommends the Honeywell Eclipse product line. They are compatible out of the box with the IoT Box. If you connect them directly to your PC they need to be configured to use your pc keyboard layout.