There is two possibilities:

Possibility 1:

You haven’t added the Owin.dll reference to the project. You can add the reference and recompile project.

Possibility 2:

You have a latest version of the dll (2.1.0 or higher), You can add below code to your web.config file. I’m using 3.0.1.0 so I have updated newVersion=”3.0.1.0″

<dependentAssembly>
<assemblyIdentity name=”Microsoft.Owin” publicKeyToken=”31bf3856ad364e35″ culture=”neutral” />
<bindingRedirect oldVersion=”1.0.0.0″ newVersion=”3.0.1.0″ />
</dependentAssembly>

You can compile the code.

I hope, its working well.

Happy Programming!!