I am getting error in C# for “ExtentHtmlReporter”

I had installed extent report but still getting below error:

Severity Code Description Project File Line Suppression State Details
Error (active) CS0234 The type or namespace name ‘Utility’ does not exist in the namespace ‘NUnitAutomationFramework’ (are you missing an assembly reference?) SeleniumTest D:MFXOneDrive – MFXchange US, IncDesktopMFX2WorkspacesC-sharp SeleniumSeleniumTestBase.cs 6

code:

using AventStack.ExtentReports;
using AventStack.ExtentReports.MarkupUtils;
using AventStack.ExtentReports.Reporter;
using NUnit.Framework;
using NUnit.Framework.Interfaces;
using NUnitAutomationFramework.Utility;
using NUnitAutomationFramework.WebElements;
using OpenQA.Selenium;
using System.Configuration;

namespace NUnitAutomationFramework.Base
{
    public class BaseSetup
    {

        public ExtentReports? extent;
        public ExtentTest? test;
        public ThreadLocal<IWebDriver> driver = new ThreadLocal<IWebDriver>();
        public ThreadLocal<ExtentTest> extent_test = new ThreadLocal<ExtentTest>();

        [OneTimeSetUp]
        public void Setup()
        {
            string? testclassfilename = TestContext.CurrentContext.Test.ClassName;
            string dir = System.Environment.CurrentDirectory;
            string? projdir = Directory.GetParent(dir)?.Parent?.Parent?.FullName;
            string reporpath = projdir + "\Reports\Report.html";
            var htmlreport = new ExtentHtmlReporter(reporpath);
            extent = new ExtentReports();
            extent.AttachReporter(htmlreport);
            extent.AddSystemInfo("Enivorment", "QA");
        }

        [SetUp]
        public void Start_Browser()
        {
            test = extent.CreateTest(TestContext.CurrentContext.Test.Name);
            extent_test.Value = test;
            SetBrowser();

            string? url = GetEnvironementData.GetEnvData();
            ActionsElements.NavigateToUrl(driver.Value, url);
            driver.Value.Manage().Window.Maximize();
        }

        public IWebDriver GetDriver()
        {
            return driver.Value;

        }

        private void SetBrowser()
        {
            string? RunEnivorment = ConfigurationManager.AppSettings["RunEnvironment"];

            if (RunEnivorment != null && RunEnivorment.Equals("Local"))
            {

                driver.Value = DriverSetup.LocalBrowserSetup(driver.Value);
            }
            else if (RunEnivorment != null && RunEnivorment.Equals("Remote"))
            {
                driver.Value = DriverSetup.RemoteBrowserSetup(driver.Value);
            }
            else
            {
                TestContext.Progress.WriteLine("Please check browser name and run enivorment value in app.config file");

            }
        }

        [TearDown]
        public void SetTestResults()
        {
            var status = TestContext.CurrentContext.Result.Outcome.Status;

            if (status == TestStatus.Failed)
            {
                var stackTrace = TestContext.CurrentContext.Result.StackTrace;
                DateTime date = DateTime.Now;
                string Filename = "Screenshot_" + date.ToString("h_mm_ss") + ".png";
                extent_test?.Value?.Fail("TestCase Status : Failed", CaptureScreenShot(driver.Value, Filename));
                extent_test?.Value?.Fail(stackTrace);
            }
            else if (status == TestStatus.Passed)
            {
                extent_test.Value.Log(Status.Pass, MarkupHelper.CreateLabel("TestCase Status : " + status, ExtentColor.Green));
            }
            extent.Flush();
            driver.Value.Quit();
        }

        public static MediaEntityModelProvider CaptureScreenShot(IWebDriver driver, string screenShotName)
        {
            ITakesScreenshot scr = (ITakesScreenshot)driver;
            var screenshot = scr.GetScreenshot().AsBase64EncodedString;

            return MediaEntityBuilder.CreateScreenCaptureFromBase64String(screenshot, screenShotName).Build();
        }

    }
}