Error In SwiftUI with a conditional binding must have a Optional Type, not string

I am making an app using Sign In With Apple, and when I put in my "on completion" part of the code, it isn't working. I've tried to put an optional "?" behind the errored part, but it isn't working. Looking around SO, I've seen similar answers, but nothing really helped. Thanks in advance :)

import SwiftUI
import CryptoKit
import AuthenticationServices
import FirebaseAuth


struct LoginView: View {
    
    @EnvironmentObject var userAuth: UserAuth
    @State var currentNonce: String
    
    
    // Adapted from https://auth0.com/docs/api-auth/tutorials/nonce#generate-a-cryptographically-random-nonce
    private func randomNonceString(length: Int = 32) -> String {
        precondition(length > 0)
        let charset: [Character] =
        Array("0123456789ABCDEFGHIJKLMNOPQRSTUVXYZabcdefghijklmnopqrstuvwxyz-._")
        var result = ""
        var remainingLength = length
        
        while remainingLength > 0 {
            let randoms: [UInt8] = (0 ..< 16).map { _ in
                var random: UInt8 = 0
                let errorCode = SecRandomCopyBytes(kSecRandomDefault, 1, &random)
                if errorCode != errSecSuccess {
                    fatalError(
                        "Unable to generate nonce. SecRandomCopyBytes failed with OSStatus \(errorCode)"
                    )
                }
                return random
            }
            
            randoms.forEach { random in
                if remainingLength == 0 {
                    return
                }
                
                if random < charset.count {
                    result.append(charset[Int(random)])
                    remainingLength -= 1
                }
            }
        }
        
        return result
    }
    
    private func sha256(_ input: String) -> String {
        let inputData = Data(input.utf8)
        let hashedData = SHA256.hash(data: inputData)
        let hashString = hashedData.compactMap {
            String(format: "%02x", $0)
        }.joined()
        
        return hashString
    }
    
    
    var body: some View {
        ZStack {
            Color.yellow
                .ignoresSafeArea()
            
            SignInWithAppleButton(
                onRequest: { request in
                    let nonce = randomNonceString()
                    currentNonce = nonce
                    request.requestedScopes = [.fullName, .email]
                    request.nonce = sha256(nonce)
                },
                onCompletion: { result in
                    switch result {
                    case .success(let authResults):
                        switch authResults.credential {
                        case let appleIDCredential as ASAuthorizationAppleIDCredential:
                            
                            guard let nonce = currentNonce else {
                                fatalError("Invalid state: A login callback was received, but no login request was sent.")
                            }
                            guard let appleIDToken = appleIDCredential.identityToken else {
                                fatalError("Invalid state: A login callback was received, but no login request was sent.")
                            }
                            guard let idTokenString = String(data: appleIDToken, encoding: .utf8) else {
                                print("Unable to serialize token string from data: \(appleIDToken.debugDescription)")
                                return
                            }
                            
                            let credential = OAuthProvider.credential(withProviderID: "apple.com",idToken: idTokenString,rawNonce: nonce)
                            Auth.auth().signIn(with: credential) { (authResult, error) in
                                if (error != nil) {
                                    // Error. If error.code == .MissingOrInvalidNonce, make sure
                                    // you're sending the SHA256-hashed nonce as a hex string with
                                    // your request to Apple.
                                    print(error?.localizedDescription as Any)
                                    return
                                }
                                print("signed in")
                                self.userAuth.login()
                            }
                            
                            print("\(String(describing: Auth.auth().currentUser?.uid))")
                        default:
                            break
                            
                        }
                    default:
                        break
                    }
                }
            )
            .frame(width: 200, height: 45, alignment: .center)
            .padding(.init(top: 400, leading: 50, bottom: 20, trailing: 200))
        }
    }
}

Declare it as optional

struct LoginView: View {

    @EnvironmentObject var userAuth: UserAuth
    @State var currentNonce: String?            // << here !!