Very good support and excellent game. The author was available for any help I needed and was patient to fix any issue I had.

So I found out this is not the project's issue. this is actually Unity's issue.

https://forum.unity.com/threads/iap-1-22-issues-with-early-unity-2019-1-releases.689647/

So I correct my rating, and wait till Unity resolves this.

Thank you Darsan for your immediate support.