You didn't stick it on the windscreen

Parking penalties are usually served by sticking them on the windscreen. It’s a myth, though, that they must be. It’s always been valid to hand them to the driver, and PCNs can be served by post under regulation 10(The Civil Enforcement of Parking Contraventions (England) General Regulations 2007 Regulation 10) when the vehicle is driven away or the CEO is prevented from serving it onto the windscreen.