Thank you Baroni! You saved my day. At first this didn't work, so at first I needed to remove the SIS from project. Then I followed your instructions above. I only unpacked the 'UnityIAP' unitypackage file at UnityPurchasing folder. There are also 'UPD' and 'UnityChannel' but are these required? I also removed the 'In App Purchasing' from Packages. Window > Package Manager. But do I need to reimport that too? Right now there are no errors.