User1644755831 posted
Hi mc9000,
1. ASP.NET Web Forms have web.config not app.config
2. As @Mike suggested try updating the package
Updates specified package for specified project
Update-Package Microsoft.Owin –Project ProjectName
http://www.nuget.org/packages/Microsoft.Owin/
3. if it doesn't work just remove and install the package again
Uninstall-Package Microsoft.Owin
Install-Package Microsoft.Owin
http://docs.nuget.org/Consume/Package-Manager-Console
Hope this works for you.
With Regards,
Krunal Parekh