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The type or namespace name "HTTP" doesn't exist in the namespace system.web error in .net 4.6.2

I am new to .net/C#. I am trying to build a project but getting a lot of missing reference error.
When I am trying to install the packages like mentioned in example, I am getting error that it already exists in the project.
PM> Install-Package Microsoft.AspNet.WebApi.Core
Package 'Microsoft.AspNet.WebApi.Core.5.2.7' already exists in project 'XXXXXXystemApi'
Time Elapsed: 00:00:16.0349189
Also, as in below screenshot, I see that there are few references which have a little warning signs which might be pointing towards what is wrong with my setup.
The type or namespace name 'Http' does not exist in the namespace 'System.Web' (are you missing an assembly reference?)
I tried to Add references to the project but I am not seeing any entry for System.Web.Http under add reference dialog.
How do I fix all this ?

Try running
dotnet restore
in the Package Manager Console
Also, unloading and reloading the project has worked for me before
(Right-click on the project name in the Solution explorer to unload and reload)

Here is what finally solved my problem.
use NuGet and downgrade a version of library you are having issue with ; then re-install the right version. Accept Licenses (which is probably what was needed). Important: close and reopen visual studio.
Reference link : solution ref

The type or namespace name 'OpenIdConnect' does not exist in the namespace 'Microsoft.Owin.Security' (are you missing an assembly reference?)

I have added a Microsoft.Owin.Security.OpenIdConnect.dll externally(tried both methods using Package Manager Console and using add reference ) to my ASP.NET MVC Application, but still get the same error.
The type or namespace name 'OpenIdConnect' does not exist in the namespace 'Microsoft.Owin.Security' (are you missing an assembly reference?)
I have attached AccountController.cs, packages.config and references
Any help.

You can try with deleting Package folder from source directory (Location: .sln file), then try to resotre packages.
Steps to restore nuget packages:
Right click on solution file.
Select Nuget restore NuGet Package option

I found the solution from Microsoft site
https://learn.microsoft.com/en-us/previous-versions/visualstudio/visual-studio-2012/0e92xd7b(v=vs.110)
Possible causes for this error include the following:
An assembly that contains the definition of a type was not referenced in the compilation; use /reference (Import Metadata) to specify the assembly
You passed a variable name to the typeof operator.
You tried to reference an assembly that is not part of your target .NET Framework profile. For more information, see Troubleshooting .NET Framework Targeting Errors.
In my case, the issue is that an assembly that is not part of my target .NET Framework profile.

Not found reference Rotativa

I want to used the Rotativa lib but not found it reference when it installed by Nuget.
I have the App_browser and Rotativa folders , but not reference to Rotativa
Why?
add the reference to Rotativa.ddl but do not compile. I have this error
The type or namespace name 'Rotativa' could not be found (are you
missing a using directive or an assembly reference?)
Sorry for my English.
Thanks.

Try to use pre-release version:
https://github.com/webgio/Rotativa/issues/177
In Nuget, mark checkbox "Include prerelises' near seach textbox.

Namespace for OData libraries v4 vs v4.5.1

I need to change the .NET framework of an existing solution from 4.5.1 to 4.0.
Several of the projects in the solution reference OData.
I have updated the references (luckilly NuGet is used here) but it seems like the namespaces are different, or I've made a glaring error somewhere.
I get the following errors when I try a build:
The type or namespace name 'Http' does not exist in the namespace 'System.Web' (are you missing an assembly reference?)
This is a typical reference list:
using System.Web.Http.OData.Builder;
using System.Web.Http.OData.Extensions;
No amount of changing references seems to help, so I'd be grateful for any suggestions!

It looks like this is the reason:
http://docs.telerik.com/data-access/developers-guide/using-web-services/asp.net-web-api/developer-guide-wcfservices-web-api-install-nuget-netframework40

Castle Windsor's DictionaryAdapter

I'm trying to upgrade one of our applications from Castle Windsor 2.1 to 2.5.
I've removed all of the Castle DLL's, and added the 2.5 version as downloaded from Castle's website:
Castle.Core.dll
Castle.Windsor.dll
Before doing the migration, I also had a reference on my project to:
Castle.Components.DictionaryAdapter.dll
However, according to Krzysztof Koźmic's post here, such reference is no longer necessary, since the DictionaryAdapter now comes bundled into Castle.Core.dll.
Upon removing it, I get errors all over the place saying:
Error 6 The type or namespace name 'DictionaryComponent' could not be
found (are you missing a using directive or an assembly
reference?)
and
Error 64 The type or namespace name 'DictionaryKey' could not be found
(are you missing a using directive or an assembly reference?)
and
Error 27 The type or namespace name 'DictionaryKeyAttribute' could not
be found (are you missing a using directive or an assembly reference?)
I then thought I'd add a reference to Castle.Components.DictionaryAdapter.dll, as it could be that CORE didn't have the types listed above implemented in it.
Doing that got rid of all the errors, but gave me a new one:
Error 8 The type 'Castle.Components.DictionaryAdapter.DictionaryAdapterFactory' exists in both 'd:.NET\app\libs\Castle.Core.dll' and 'd:.Net\app\libs\Castle.Components.DictionaryAdapter.dll'
The error happens on the following method implementation:
internal static ISettingService GetServiceFromAdapterFactory(NameValueCollection collection)
{
var adapter = new DictionaryAdapterFactory();
return adapter.GetAdapter<ISettingService>(collection);
}
Has anyone faced such problem when upgrading from 2.1 to 2.5 and could offer me some advice?
Thanks in advance.

To answer my own question here, this is what I had to change in my code.
Changed all the DictionaryKey to become KeyAttribute
Changed all the DictionaryComponent to become ComponentAttribute
Used Kernel.Register instead of Kernel.AddComponent
And obviously removed all the 2.1 DLL's and added the 2.5. You will notice that the DictionaryAdapter.dll has been removed, as it's now bundled within Castle.Core.dll
Hope that helps someone in the future.