Selenium does not work after setup

Selenium can be sensitive to different environments, versions, and configurations. You error could be due to a misconfiguration or incorrect setup. As you did not provide full code, I am guessing as to whether you have imported the correct files/namespaces -

//Import necessary namespaces...
using OpenQA.Selenium;
using OpenQA.Selenium.Chrome;

//Create a ChromeDriverService with default settings...
var driverService = ChromeDriverService.CreateDefaultService();

//Hide the command prompt window of the ChromeDriver...
driverService.HideCommandPromptWindow = true;

//Create a ChromeDriver instance with the configured service and options...
var driver = new ChromeDriver(driverService, new ChromeOptions());

//Maximize the browser window...
driver.Manage().Window.Maximize();

//Navigate to your specified URL...
driver.Navigate().GoToUrl("https://***************.aspx");

//Find the dropdown element by its name attribute...
IWebElement MisafirDropDownElement = driver.FindElement(By.Name("ddlMisafirSayisi"));

//Create a SelectElement instance to work with the dropdown...
SelectElement SelectAnMisafir = new SelectElement(MisafirDropDownElement);

//Select an option from the dropdown based on the text in comboBox1...
SelectAnMisafir.SelectByText(comboBox1.Text.Trim());

Some other checks you can perform -
Make sure that the ChromeDriver executable is in a directory that is included in your system's PATH environment variable. You might need to specify the full path to the ChromeDriver executable in the ChromeDriverService.CreateDefaultService() call.

Make sure that your Chrome browser and ChromeDriver version are compatible. If you've recently updated your browser, you might need to update your ChromeDriver as well.

Check that you've installed the necessary NuGet packages for Selenium and ChromeDriver in your project.

Ccheck if your firewall or other security software is maybe blocking the execution of ChromeDriver.

Try running the code in debug mode to check for errors or issues.

I have  solved the problem by adding this line by myself.. 

string path = "";
var driverService = ChromeDriverService.CreateDefaultService(path);