Do you have the .NET example working using Visual Studio? I always get the samples working first, using the technologies mentioned (VS versus MonoDevelop for example). Once I get it working, THEN I customize to my requirements/IDE.
I had the same problem, but I was able to get the Google Sheets API working in Unity 2018.1.9f1.
When I tried to use their “netstandard2.0” DLL’s with Scripting Runtime Version .NET 3.5, it didn’t work and the properties of their dll’s showed an error indicating they required .NET 4.x.
When I used the “net45” versions of their DLL’s and Scripting Runtime Version .NET 4.x it worked.