Why I can't scan QR code inside Whatsapp?

I have Whatsapp latest version & have the Whatsapp Web Option I put it to scan but it just doesn't scan it. I Installed a QR Scanner app and it works perfectly. I uninstalled the QR Scanner app to see if that was the issue and still doesn't work. My phone is a Sony Xperia E1 Dual... anyone help me...