On Unity 2018.3. Got this error after importing Monetization SDK 3.0. Ad services where on but I turned them off before importing. I reimported all assets but it got worst since many errors where reported with TMP examples. Deleted TMP examples folder in project but same errors. Deleted all tmp examples refs in Assembly-CSharp.csproj then got another error about loading this file (my text editor was closed). I restarted my computer and guess what? Everything now is fine!?!?!?! Hope this will help someone. Edit: jut saw this post from Unity reporting the same error but in my situation, I installed SDK 3.0 AFTER using 2018.3 for a few weeks. Edit2: It came back 2 weeks later while working on a script in Visual Studio that has no relation with ads. In fact didn't touch the ad system for the hole time. It was working fine. Couldn't fix it. Had to restore a previous version.