I am implementing in-app purchase using https://github.com/anjlab/android-inapp-billing-v3 . But while in app pop up open it shows \"Error Authentication is required.You nee
In my case what happened was that the right product was all set up at console, however it wasn't activated.

In my case my base64EncodedPublicKey was incorrect and also my inapp product was inactive. Be sure ur inapp product id is same as in the console.
I struggled with this error for hours. My product IDs were correct, app was published to Alpha, I activated the Alpha testers list, joined through the opt-in link but nothing was working.
In the end I discovered that you cannot use the same Gmail account that is used for the Developer Console. That was it, added another gmail account as a tester and used it for downloading the app and no more authentication errors.
Also, just a note making changes to the app and building a debug version caused the error to re-appear. Only signed release APKs can be used.
Make sure you build and sign the application with the same Keystore that you signed the APK that was uploaded to Google Play Beta channel. I made the mistake of signing with different Keystores, and didn't notice it.
If anybody getting the above popup you can re-check through the following steps because unfortunately this google popup is not enough informative for a clue.
Hope this helps :)
Solved, It was only mistake of product Id, The product Id from Play-store and in app SKU_NAME should be same for in app billing.
It is not mention properly in iab sample project which one is productId
i.e SKU_GAS ="gas" and SKU_PREMIUM="premium" in project
then
"gas" and "premium" must be available on play-store as productId.