After upgrading my Telerik Ajax version from 2020.3.915.45 to the latest 2021.2.511.45 version, I am now receiving this error, in my solution. The error occurred right after I tried to upgrade Telerik from inside Visual Studio. BTW, I do not have this file: web.config.transform I am also getting a similar error when I go to this:
Configure Telerik section in Visual Studio (see screenshot).

Failed to execute command.

System.IO.FileNotFoundException: Could not find file 'C:\Git\nedwneappdev\Web-Online_Offline_Modems\web.config.transform'.
File name: 'C:\Git\nedwneappdev\Web-Online_Offline_Modems\web.config.transform'
   at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)
   at System.IO.FileStream.Init(String path, FileMode mode, FileAccess access, Int32 rights, Boolean useRights, FileShare share, Int32 bufferSize, FileOptions options, SECURITY_ATTRIBUTES secAttrs, String msgPath, Boolean bFromProxy, Boolean useLongPath, Boolean checkHost)
   at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize)
   at System.Xml.XmlDownloadManager.GetStream(Uri uri, ICredentials credentials, IWebProxy proxy, RequestCachePolicy cachePolicy)
   at System.Xml.XmlUrlResolver.GetEntity(Uri absoluteUri, String role, Type ofObjectToReturn)
   at System.Xml.XmlTextReaderImpl.OpenUrlDelegate(Object xmlResolver)
   at System.Threading.CompressedStack.runTryCode(Object userData)
   at System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode code, CleanupCode backoutCode, Object userData)
   at System.Threading.CompressedStack.Run(CompressedStack compressedStack, ContextCallback callback, Object state)
   at System.Xml.XmlTextReaderImpl.OpenUrl()
   at System.Xml.XmlTextReaderImpl.Read()
   at System.Xml.XmlLoader.Load(XmlDocument doc, XmlReader reader, Boolean preserveWhitespace)
   at System.Xml.XmlDocument.Load(XmlReader reader)
   at Telerik.VSX.Web.Helpers.ExistingWebConfig.Load(XmlReader reader)
   at System.Xml.XmlDocument.Load(String filename)
   at Telerik.VSX.Web.Helpers.ExistingWebConfig.Load(String filename)
   at Telerik.VSX.Web.Helpers.TelerikConfiguredWebConfig.Retrieve(IProjectWrap project, Boolean create, String relativeProjectPath)
   at Telerik.VSX.Web.Helpers.TelerikConfiguredWebConfig.Retrieve(IProjectWrap project, Boolean create)
   at Telerik.Web.UI.VSX.Helpers.ProjectSettingsReader..ctor(IProjectWrapUIComponents project)
   at Telerik.Web.UI.VSX.ProjectConfigurators.ProjectConfigurationTypeSelector.GetProjectSettingsReader()
   at Telerik.Web.UI.VSX.ProjectConfigurators.ProjectConfigurationTypeSelector.GetProjectConfigurationPreWizardData(IScenarioDef template)
   at Telerik.Web.UI.VSX.ProjectConfigurators.ProjectConfigurationTypeSelector.ProjectConfiguration()
   at Telerik.Web.UI.VSX.ProjectConfigurators.StreamlinedProjectConfiguration.StartConfigureWizard()
   at Telerik.Web.UI.VSPackage.CommandExecutor.Execute(UInt32 commandId)
   at Telerik.VSX.VSPackage.PackageBase.CommandCallback(Object sender, EventArgs e)

RadCodeBLock and RadScriptBlock is not able to handle the server tags for all case scenarios, hence RadAjaxManager throws the exception mentioned in the title.