Unity Monetization SDK error when importing from the asset store, [Common Solution].
Here are two ways to fix the common error that most people get when they import the Unity Monetization SDK from the unity asset store.
(You likely installed from the asset store and package manager. You should only install from 1 source, and ideally you should switch to package manager so that its easy to update in the future.)
If using the "Unity Monetization SDK" then:
Deleted the Unity Ads package from my Assets folder.
Removed the Advertisement package (Window / Package Manager / Advertisement / Remove).
Go back to the Asset Store and imported the "Unity Monetization" package again.