Cannot Build with UCB, Error related to Purchasing and UI? Unity 2019.3.5

 Error: Could not load signature of UnityEngine.Purchasing.UIFakeStore:GetOkayButton due to: Could not load file or assembly 'UnityEngine.UI, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' or one of its dependencies. assembly:UnityEngine.UI, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null type:<unknown type> member:(null) signature:<none>

Hello everyone. This error comes up in my editor after I clear my library and relaunch the editor. In editor it isn't really a problem. But it also appears in Cloud Build for Android and IOS and stops the builds from succeeding.

I recently upgraded a project to Unity 2019.3.5, and updated the IAP package to 1.23.1. I was forced to update because my previous IAP version of 1.22.0 also had the issue.

I have looked at the Unity Forums and a question about this issue from months ago and it doesn't seem any posted solution can solve this for me.

My attempts to solve this:

• Removed Packages and Assets completely and then reinstalled them.

• Used Reimport All

• Removed the Demo content, IAPButton and related scripts that I don't use.

• Removed the UIFakeStore prefab.

Any additional ideas, or even a concert solution would be appreciated.