Xcode – What is the solution to the error "The value of type [User] 'cannot be converted to the expected argument type' User"?

In my page named service, xcode points to the user and gives an error. but it doesn’t work. What do you think should I change?
my user is already optional. I think it is an index problem but I don’t know how to solve it I would appreciate it if you could help.where do you think the problem

message.swift

import Firebase

struct Message {
    let text: String
    let toId: String
    let fromId: String
    var timestamp: Timestamp!
    var user: User?
    let isFromCurrentUser :Bool
    
    init(dictionary: [String: Any]) {
        self.text = dictionary["text"] as? String ?? ""
        self.toId = dictionary["toId"] as? String ?? ""
        self.fromId = dictionary["fromId"] as? String ?? ""
        self.timestamp = dictionary["timestamp"] as? Timestamp ?? Timestamp(date: Date())
        self.isFromCurrentUser = fromId == Auth.auth().currentUser?.uid
    }
    
    
}

struct Conversation {
    let user: User
    let message : Message
}

Service.Swift

import Firebase
 
struct Service  {
    static func  fetchUsers (completion: @escaping([User]) -> Void) {
        var users = [User] ()
       COLLECTION_USERS.getDocuments { (snapshot, error) in
            snapshot?.documents.forEach({ (document) in
               
                let dictionary  = document.data()
                let user = User(dictionary: dictionary)
                users.append(user)
                completion(users)
            
            })
        }
    }
    
    static func fetchUser(widhtUid uid: String, completion:@escaping([User]) ->Void) {
        
COLLECTION_USERS.document(uid).getDocument { (snapshot, error) in
    guard let dictionary = snapshot?.data() else {return}
    let user = User(dictionary: dictionary)
    completion(user)
        }
        
        
    }
    
    
    static func fetchConversations (completion: @escaping([Conversation]) ->Void) {
        var conversations = [Conversation]()
        guard let uid = Auth.auth().currentUser?.uid else {return}
        
        let query = COLLECTION_MESSAGES.document(uid).collection("recent-messages").order(by:  "timestamp")
        query.addSnapshotListener { (snapshot, error) in
            snapshot?.documentChanges.forEach({ change in
                let dictionary = change.document.data()
                let message = Message(dictionary: dictionary)
                
                self.fetchUser(widhtUid: message.toId) { user in
                    let conversation = Conversation(user:user, message: message)
                    conversations.append(conversation)
                    completion(conversations)
                }
                
           
            })
        }
        
    }
    
    
    static func fetchMessages    (forUser user: User, completion: @escaping([Message])-> Void)  {
    var messages  = [Message]()
    guard let currentUid = Auth.auth().currentUser?.uid else {return}
        let query = COLLECTION_MESSAGES.document(currentUid).collection(user.uid).order(by: "timestamp")
        query.addSnapshotListener{(snapshot,error) in
        snapshot?.documentChanges.forEach({ change in
            if change.type == .added {
                let dictionary = change.document.data ()
                
                messages.append(Message(dictionary: dictionary))
                completion(messages)
            }
        
            
            
            
        })
    }
    }
 static func  uploadMessage(message: String, to user: User, completion: ((Error?)->Void)?) {
        guard let currentUid = Auth.auth().currentUser?.uid else {return}
        let data = ["text": message,
                    "fromId": currentUid,
                    "toId": user.uid,
                    "timestamp" : Timestamp(date: Date())] as [String : Any]
           COLLECTION_MESSAGES.document(currentUid).collection(user.uid).addDocument(data:data) { _ in
                COLLECTION_MESSAGES.document(user.uid).collection(currentUid).addDocument(data:data,completion:completion)
            
            COLLECTION_MESSAGES.document(currentUid).collection("recent- messages").document(user.uid).setData(data)
            
            COLLECTION_MESSAGES.document(user.uid).collection("recent- messages").document(currentUid).setData(data)
            
            
        }
    }
}