Link to task <> states that it is defined in mscorlib

Question

I have a .net 4.5.2 Winforms application that uses OAuth 2.0 and Google DoubleClickBidManager to make some API calls.

I need to port it to .net 3.5 so that I can pack it in SSIS.

The google APIs use Async, Await, and Task, and to work in 3.5, I added this nuget package and fixed many error links.

But I can not fix this:

Can someone please give me ideas how to fix this?