I am doing User authentication in MVC using web API 2, OWIN.

Hello dip

Make sure you've installed OWIN from NuGet package manager.

Open your Project within the Solution Explorer

Right-click the References folder and Choose Add Reference....

Find and select the Microsoft.Owin library or assembly from the list provided

Click the Ok button to add it to your Project.

You should then be able to reference the Microsoft.Owin namespace within your code.

Please check these Links

https://stackoverflow.com/questions/22060716/signalr-2-0-error-could-not-load-file-or-assembly-microsoft-owin-security