QQ does have a function to scan the QR code. In fact, almost all Chinese social apps have the Scanning feature. In QQ app, you can click the top right “+” plus icon and then find a “Scan” button in the popup menu.
As far as I know, the most common reason is that your phone number might probably have been used to register too many QQ accounts, might not by you but by the previous phone number owners. There is probably no