The type or namespace name 'Purchasing' does not exist!

History:
I have a working 2019.1.0f2 project with the package “In App Purchasing V 2.06” plus an adapted “Purchaser.cs” file. All works!

Present:
Now I start a new project using Unity 2019.3.4f1 and imported the same V2.06.
Errors:

The type or namespace name 'Security' does not exist in the namespace 'UnityEngine.Purchasing' (are you missing an assembly reference?)

So, I remove the package and look into the Service Tab.
I click “Reimport”:

Assets/System/FeatureManager.cs(5,19): error CS0234: The type or namespace name 'Purchasing' does not exist in the namespace 'UnityEngine' (are you missing an assembly reference?)

When I check the folder plugins/UnityPurchasing it shows 2 packages, so I import those.

this brings new errors.

  Error: Could not load signature of UnityEngine.Purchasing.FakeUDPExtension:GetUserInfo due to: Could not load file or assembly 'UDP, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null' or one of its dependencies. assembly:UDP, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null type:<unknown type> member:(null) signature:<none>

    Assembly 'Assets/Plugins/UnityPurchasing/Bin/Editor.dll' will not be loaded due to errors:
    Unable to resolve reference 'UDP'. Is the assembly missing or incompatible with the current platform?
    Reference validation can be disabled in the Plugin Inspector

Assembly 'Assets/Plugins/UnityPurchasing/Bin/Stores.dll' will not be loaded due to errors:
Unable to resolve reference 'UDP'. Is the assembly missing or incompatible with the current platform?
Reference validation can be disabled in the Plugin Inspector.

when I import the files from plugins/UnityPurchasing again, editor.dll and stores.dll are shown as changed. So import again and again and again.

so what´s going on?

I deleted all related files and started over again by switching on the IAP. After import this time a little installer helper window showed up which installed everything in order. So it seems to be correct now.

The upcoming Google / Apple Tangle error could be fixed by runing that obfuscator thingy in the window > iap menu.