I had the same issue and I think it's solved now.
Open AdMobDependencies.cs file, located inside of Assets -> GoogleMobileAds -> Editor, with your text editor and find the line in SetupDeps() method:
Google.VersionHandler.InvokeInstanceMethod(svcSupport, "DependOn", new object[] { "com.google.android.gms", "play-services-ads", "LATEST" },
Now change the string "LATEST" to "10.2.1". That worked for me, hope it helps someone.