Error Domain=NSCocoaErrorDomain Code=3840 "JSON text did not start with array or object and option to allow fragments not set


#1

@IBAction func postRequest(_ sender: Any) {

    guard let url = URL(string: "http://185.x.xx.xxxx/api/topback") else { return }
    
    let userData = ["message": "Networking"]
  
    var request = URLRequest(url: url)
    request.httpMethod = "POST"
    request.addValue("application/json", forHTTPHeaderField: "Content-Type")
    request.addValue("application/json", forHTTPHeaderField: "Accept")
    
    guard let httpBody = try? JSONSerialization.data(withJSONObject: userData, options: []) else { return }
    request.httpBody = httpBody
    
    let session = URLSession.shared
    session.dataTask(with: request) { (data, response, error) in
        
        guard let response = response, let data = data else { return }
        
        print(response)
        
        do {
            let json = try JSONSerialization.jsonObject(with: data, options: [])
            print(json)
        } catch {
            print(error)
        }
    } .resume()
}

#2

В вашем заголовке указана суть проблемы.
Смотрите на ответ json который получаете, там неверный формат или что-то еще.