Could not load file or assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51' or one of its dependencies

When you have finished your website and you want to deploy it on the server, if you get an error 'Could not load file or assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51' or one of its dependencies. The system cannot find the file specified.'

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.IO.FileNotFoundException: Could not load file or assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51' or one of its dependencies. The system cannot find the file specified.

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Assembly Load Trace: The following information can be helpful to determine why the assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51' could not be loaded.

WRN: Assembly binding logging is turned OFF.
To enable assembly bind failure logging, set the registry value [HKLM\Software\Microsoft\Fusion!EnableLog] (DWORD) to 1.
Note: There is some performance penalty associated with assembly bind failure logging.
To turn this feature off, remove the registry value [HKLM\Software\Microsoft\Fusion!EnableLog].

Stack Trace:

[FileNotFoundException: Could not load file or assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51' or one of its dependencies. The system cannot find the file specified.]
   System.ModuleHandle.ResolveMethod(RuntimeModule module, Int32 methodToken, IntPtr* typeInstArgs, Int32 typeInstCount, IntPtr* methodInstArgs, Int32 methodInstCount) +0
   System.ModuleHandle.ResolveMethodHandleInternalCore(RuntimeModule module, Int32 methodToken, IntPtr[] typeInstantiationContext, Int32 typeInstCount, IntPtr[] methodInstantiationContext, Int32 methodInstCount) +111
   System.ModuleHandle.ResolveMethodHandleInternal(RuntimeModule module, Int32 methodToken, RuntimeTypeHandle[] typeInstantiationContext, RuntimeTypeHandle[] methodInstantiationContext) +63
   System.Reflection.CustomAttributeData..ctor(RuntimeModule scope, CustomAttributeRecord caRecord) +62
   System.Reflection.CustomAttributeData.GetCustomAttributes(RuntimeModule module, Int32 tkTarget) +107
   System.Reflection.CustomAttributeData.GetCustomAttributesInternal(RuntimeAssembly target) +69
   System.Reflection.RuntimeAssembly.GetCustomAttributesData() +9
   Owin.Loader.DefaultLoader.SearchForStartupAttribute(String friendlyName, IList`1 errors, Boolean& conflict) +143
   Owin.Loader.DefaultLoader.GetDefaultConfiguration(String friendlyName, IList`1 errors) +46
   Owin.Loader.DefaultLoader.LoadImplementation(String startupName, IList`1 errorDetails) +75
   Owin.Loader.DefaultLoader.Load(String startupName, IList`1 errorDetails) +21
   Microsoft.Owin.Host.SystemWeb.OwinBuilder.GetAppStartup() +115
   Microsoft.Owin.Host.SystemWeb.OwinHttpModule.InitializeBlueprint() +28
   System.Threading.LazyInitializer.EnsureInitializedCore(T& target, Boolean& initialized, Object& syncLock, Func`1 valueFactory) +115
   Microsoft.Owin.Host.SystemWeb.OwinHttpModule.Init(HttpApplication context) +106
   System.Web.HttpApplication.RegisterEventSubscriptionsWithIIS(IntPtr appContext, HttpContext context, MethodInfo[] handlers) +536
   System.Web.HttpApplication.InitSpecial(HttpApplicationState state, MethodInfo[] handlers, IntPtr appContext, HttpContext context) +173
   System.Web.HttpApplicationFactory.GetSpecialApplicationInstance(IntPtr appContext, HttpContext context) +218
   System.Web.Hosting.PipelineRuntime.InitializeApplication(IntPtr appContext) +296

[HttpException (0x80004005): Could not load file or assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51' or one of its dependencies. The system cannot find the file specified.]
   System.Web.HttpRuntime.FirstRequestInit(HttpContext context) +10077800
   System.Web.HttpRuntime.EnsureFirstRequestInit(HttpContext context) +95
   System.Web.HttpRuntime.ProcessRequestNotificationPrivate(IIS7WorkerRequest wr, HttpContext context) +254

Try to install new version net framework, then restart your computer.

If you still get the same error, you can open the web.config file, then add netstandard to you configuration as shown below.

<system.web>
  <compilation debug="true" targetFramework="4.7.1" >
    <assemblies>
      <add assembly="netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51"/>
    </assemblies>
  </compilation>
  <httpRuntime targetFramework="4.7.1" />
</system.web>

And don't forget to add a reference to netstandard library from your project, 

Depending on the Visual Studio version you have installed it will be somewhere like C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\MSBuild\Microsoft\Microsoft.NET.Build.Extensions\net461\lib\netstandard.dll

Next, set the Copy Enable to true in netstandard.dll property. You can do that, by open your Solution Explorer and right click on netstandard.dll then set the Copy Local property to true.