A script of my asset has an error

I just downloaded an Asset from here and I got this error.

Assets/SaveGameFree/Scripts/SaveGameWeb.cs(580,19): error CS1061: Type `UnityEngine.Networking.UnityWebRequest' does not contain a definition for `isNetworkError' and no extension method `isNetworkError' of type `UnityEngine.Networking.UnityWebRequest' could be found. Are you missing an assembly reference?

And my IDE points out that the error was just right here on this line of code:

I am totally confused why this assets works on the others and not mine. It was also declared that:

Package has been submitted using Unity 5.6.0, and 2017.1.0 to improve compatibility within the range of these versions of Unity.

I’m using Unity 5.6.1 Personal.

It would be much appreciated if you can help me with this.

I believe “isNetworkError” and “isHttpError” is deprecated.
Try using m_Request.isError instead.