If you are using both of ASP.NET MVC and ASP.NET Web API in your application, you need to add Abp.Owin nuget package to your project.

Installation

Add Abp.Owin nuget package to your host project (generally, to the Web project).

Install-Package Abp.Owin

Usage

Then call UseAbp() extension method in your OWIN Startup file as shown below:

[assembly: OwinStartup(typeof(Startup))]
public class Startup
{
    public void Configuration(IAppBuilder app)
    {
        app.UseAbp();

        //your other configuration...
    }
}