Because OnJumped is a BlueprintNativeEvent, you need to implement OnJumped_Implementation in your derived class:
H:
virtual void OnJumped_Implementation() override;
CPP:
void AMyCharacter::OnJumped_Implementation()
BlueprintNativeEvent functions cause additional code to be generated and inserted into the ACharacter base class, so you won’t see a declaration in the base classes header and can’t override the OnJumped() function directly. You can however see the base implementation in ACharacter.cpp (which at the moment is empty).