Unable to list folder files in iOS Swift

I am facing below error when I am trying to list folder or get current account info. Please help

Error Domain=NSURLErrorDomain Code=-999 "cancelled" UserInfo={NSErrorFailingURLKey=https://api.dropbox.com/2/users/get_current_account, NSLocalizedDescription=cancelled, NSErrorFailingURLStringKey=https://api.dropbox.com/2/users/get_current_account}

Error Domain=NSURLErrorDomain Code=-999 "cancelled" UserInfo={NSErrorFailingURLKey=https://api.dropbox.com/2/files/list_folder, NSLocalizedDescription=cancelled, NSErrorFailingURLStringKey=https://api.dropbox.com/2/files/list_folder}

Below the code I have tried. 

let client = DropboxClient(accessToken: "MYTOKEN")

        client.users.getCurrentAccount().response { (response, error) in

            if let result = response {

                print(result)

            } else if let error = error {

                print(error)

            }

        }

        client.files.listFolder(path: "").response(queue: DispatchQueue(label: "MyCustomSerialQueue")) { response, error in

            if let result = response {

                print(Thread.current)  // Output: <NSThread: 0x61000007bec0>{number = 4, name = (null)}

                print(Thread.main)     // Output: <NSThread: 0x608000070100>{number = 1, name = (null)}

                print(result)

            } else if let error = error {

                print(error)

            }

        }