将代码更改为以下内容:

Change your code to the following:

// Add a new document with a generated ID
var ref: DocumentReference? = nil
ref = db.collection("users").addDocument(data: [
    "firstname": firstname,
    "lastname": lastname,
    "uid": result!.user.uid
]) { err in
    if let err = err {
        print("Error adding document: \(err)")
    } else {
        print("Document added with ID: \(ref!.documentID)")
    }
}

使用此打印语句 print(添加文档时出错:\(err)")您可以确切地知道错误是什么.

Using this print statement print("Error adding document: \(err)") you can know exactly what the error is.

还将您的安全规则更改为以下内容:

Also change your security rules to the following:

// Allow read/write access to all users under any conditions
// Warning: **NEVER** use this rule set in production; it allows
// anyone to overwrite your entire database.
service cloud.firestore {
  match /databases/{database}/documents {
    match /{document=**} {
      allow read, write: if true;
    }
  }
}