cannot compile on monterey macos

[sanruby77]

class IdentityCert : public IdentityCertBase {
public:
explicit IdentityCert(std::string identity);
~IdentityCert() override = default;

std::string getIdentity() const override;

std::optionalstd::string getDER() const override;

private:
std::string identity_;
}; why the compiler complain the error "cannot use override for the std::optionalstd::string getDER() const override;"