Argentina: "Your current digital account setting does not allow you to subscribe for Kindle Unlimited on amazon.com. Please contact customer service to learn about Kindle Unlimited availability in your local Amazon website."
Kindle Unlimited on the Amazon website is strictly for US residents. And there is no local Amazon website for Argentina, so a subscription to Kindle Unlimited is not possible. Unfortunately, there is no solution if you live in Argentina except to wait until there there is a local website. This could be quite a while in the future as Amazon only adds one or two local Kindle stores a year.