I am getting this kind of error message when I try to set up an In-app purchase within my Unity project.
I have already set up in-app purchases 100 times for my various project but this time, I am getting this kind of error message and I can’t able to continue my process.
The in-app purchase feature is already ON in the services section and I have attached a screenshot too so you become clear.
But again and again, I was getting this kind of error message in the editor.
As you describe, it’s probably not a Unity bug if it has worked for you in the past. Please compare by adding IAP to a brand new project. Specifically, I believe you may have accidentally removed the Purchasing package in Package Manager.
This is complete new project and I just linked the project with Unity Services and enabled in-app purchase.
For this project, first time I am importing Unity IAP package because of this reason I written this might be Unity bug.
Because with the dialog box, you are seeing, I have already enabled the in-app section then also dialog is suggesting me to enable the in-app section within the services.
I have reopened the Unity and also tried to enable-disable in-app services but nothing worked.
Okay, I can able to find a solution by installing in-app purchase package through Package Manager.
After this installation above problem gets to solve and I can able to import Unity IAP plugin data.
@JeffDUnity3D you have not mentioned about this step within your documentation.
It is typically not necessary, I have never needed to install it separately, nor have our testers. You must have removed that package somehow. What version of Unity? And you said IAP has worked before, what are you doing differently?
Into these - not just me but other developers also facing the problem:
I found the solution from this post.
Unity Version 2018.4.9f1 - windows version and just downloaded the latest Unity IAP package and try to do the setup.
Works for me, so I’m unable to reproduce. Glad you found your solution!
@JeffDUnity3D thank you, Sir, for your quick attention and I am happy that you are taking care of single Unity developer too 
Thanks Its Work for me.
