The type or namespace IAppBuilder could not be found(missing using a directive pr an assembly reference)

Try to use Package Manage Console and do

Update-Package Owin -Reinstall

I was having similar issue. But instead Owin, problem was causing Microsoft.Owin, obviously

Update-Package Owin -Reinstall

Didn't work, neither did Update-Package Owin

BUT

Install-Package Microsoft.Owin

did work fine for me, thanks.

The IAppBuilder interface is found under Owin package. Just add a reference in your class file:

using Owin;

And rebuild. Your project will pick this up.

I have no idea why VS didn't pick this up, but it didn't. Once I added this reference to my project, then everything fell into place.