Task' does not contain a definition for 'Content'

Hi, how to overcome the error below?
I have follow below tutorial but I get the red line that state that doest not contain definition of Content.

'Task' does not contain a definition for 'Content' and no extension method 'Content' accepting a first argument of type 'Task' could be found (are you missing a using directive or an assembly reference?)

This is the error but any idea to solve it?

I have add this Microsoft.AspNet.WebApi.Client package but still fail