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 through here. For more information, see the documentation on Conditional Compilation.