StringProtocol 관련 오류

Referencing operator function '' on 'StringProtocol' requires that 'Subscribers.Completion<AFError>' conform to 'StringProtocol'

func login(email: String, password: String, autoLogin: Bool) {
    ApiService.login(email: email, password: password, autoLogin: autoLogin)
        .sink { (completion:Subscribers.Completion<AFError>) in
            print("ApiVM completion: \(completion)")
            if completion == "failure" {
            	print("오류")
            }
        } receiveValue: { (receivedUser: LoginInfoRes) in
            print("ApiVM 테스트")
            self.loginReturn = receivedUser.user
            self.tokenReturn = receivedUser.token
            self.apiAccessReturn = receivedUser.apiAccess
            self.loginSuccess.send()
        }.store(in: &subscription)
}

5번줄 처럼 하게되면 오류가 난다.

다음과 같이 처리

switch completion {
case .failure:
    print("실패")
    break;
case .finished:
    print("end")
}