Assets/Scripts/IAPManager.cs(89,28): error CS1061: ‘IGooglePlayStoreExtensions’ does not contain a definition for ‘SetDeferredPurchaseListener’ and no accessible extension method ‘SetDeferredPurchaseListener’ accepting a first argument of type ‘IGooglePlayStoreExtensions’ could be found (are you missing a using directive or an assembly reference?)
I then updated to the latest version (4.0.3) of IAP in the package manager but the error persists.
No, you would not want to install an old version of IAP, we have fixed many issues since then. You will want to use IAP 4.1.3 (or higher). My last post shows the correct and updated syntax.