Why do I get a "Advertisements" does not exist in the namespace "UnityEngine" error"?

This error is typically caused by one of two issues:

• You are building for a platform that is not supported by Unity Ads
• You have not imported the Unity Ads package into your project yet.

To resolve the problem:

• Ensure that you have installed Unity Ads by using the Package Manager 
• Ensure that you have selected iOS or Android as a build target (File > Build Settings).
• If you are building your Project for an unsupported platform, you can add a UNITY_ADS conditional define that allows you to build with Unity Ads without affecting other platforms.

If you continue to see this error after trying these suggestions, open a support ticket by emailing unityads-support@unity3d.com. For more information, see the documentation on Conditional Compilation.