Monetization breaks iOS build in XCode. Also breaks Android build.

I am trying to incorporate Monetization (primarily Ads) into my game, however as soon as I add it I get this error while trying to build in XCode:

I also get this error message when trying to build for android after adding Monetization.

I’m a little late and you’ve likely found the solution already. But I’ll post a response as I just worked through this same issue.

If you expand to the full log message, the problem occurs because of duplicate UnityAd.dlls (or duplicate namespace, or duplicate UnityAd something). The imported asset is conflicting with the default/included UnityAds.

To resolve, deactivate the “built-in” UnityAds to enable the imported asset as your sole UnityAd:
(Services → Ads → Advanced → DISABLE built-in extensions)

This worked for my build in 2018.1