​”Samsung account already exists in account manager”.

I had the same problem. The fix is pretty simple for Android Marshmallow 6.0? Just do the following:

Go to Settings 》Applications 》

Application Manager 》 Select Kindle 》

Now here you will find “Permissions” click on that. 》 Grant all 3 permissions I.e Contact, Storage and Telephone . 

Now go to your Kindle by Samsung and click Samsung Book Deals and Voila!

http://www.wmsr.com/static/Specials%20&%20Deals

Larry bday idea. Have to schedule sooon.