Hello,
I finally got Unity Ads 2.0 to work in my project using Unity 5.6, what I did is a work around but it does the job well. Since i am a not a programmer I didn't know the proper code to adjust the existing ecosystem Unity Ads actions but I had found out where the problem was, and here is what I did.
1. Using the built in extension didn't seem to work, so you have to uncheck the "Enable Built In Extention" and download the Unity Ads package from the Asset Store
https://www.assetstore.unity3d.com/en/#!/content/661232. Download the Unity Ads actions from the Ecosystem, edit the scripts and remove any line that starts with #if or # else. The problem seems to be no connection between these commands and Unity itself, removing them brought the actions back to life.
3. Use the action "Platform dependent event", that's available on the ecosystem to choose which action to trigger whether it is android or iOS incase you are targeting more than one platform. all you have to do is initialize the target platform at the start of the game, later on all ads actions will respond accordingly.
I hope this helps incase it hasn't been figured out yet