Google Play In-app Billing V3 - Error - Authentication Is Required

I have looked at other similar questions on SO regarding the same issue and all point to the same point. To check the ID of the product. I am implementing in-app purchases for the

Solution 1:

Make sure your .apk with billing in place (and right permissions) is Published (be it alpha) and allow some time for Google Play to absorb the new .apk, may be a couple of hours.

Also, make sure the application version code in your current development build is the same as the one of the published billing-enabled build.

Solution 2:

On upload APK page in ALPHA TESTING there is a link "Opt-in URL ". Go there and accept being a tester using the testing account. Aso, here is a useful checklist https://stackoverflow.com/a/22469253/1819570