Error CS0234. Missing "UnityEditor.iOS.Xcode.Extensions". Any ideas on how to fix it? :(

Hi, I recently upgraded the OneSignal plug-in on my project and now it refuses to compile, giving me the following error...

Assets/Plugins/OneSignal/Editor/PostProcessBuildPlayer_iOS.cs(9,29): error CS0234: The type or namespace name `Extensions' does not exist in the namespace `UnityEditor.iOS.Xcode'. Are you missing an assembly reference?

I've scoured the internet for a solution, and tried everything I've found, but nothing seems to fix it.
Any suggestions would be most welcome.

FYI - I am using Unity 5.6.6f2 (the project has been going on for a few years!)

Thanks in advance for your help.
Cheers,
Dan