First win the buy box than add to cart option will be visible:

To win the Buy Box, first make sure that both you and your listed items are eligible.

Becoming eligible to win the Buy Box doesn’t guarantee that you will win it; that said, there are a few things you can do to increase your chances.

I. Price your items competitively
There are a few of ways to check and make sure you’re pricing your items competitively.

Use Automate Pricing to price your products competitively by matching or beating competitive offerings and increasing the chances of winning the Buy Box
Visit the Manage Pricing page where the Buy Box Price column shows the winning offer, even if it isn’t yours.
See the Pricing Dashboard for real-time data about your prices and how they compare to other offers.
View your pricing recommendations in the Selling Coach Pricing Report.
II. Offer Prime and free shipping
Sellers who offer faster shipping options are more likely to win the Buy Box. You can look up your available options in Shipping Settings.

Note: Only items that can be shipped to the customer’s shipping address are eligible to win the Buy Box.
III. Provide great customer service
We measure customer service in several different ways, and you can check your Account Health to see how you’re doing. While certain metrics such as Order Defect Rate, Cancellation Rate, and Late Dispatch Rate may weigh more heavily when considering Buy Box status, we consider all the available metrics.

Staying tuned in to all aspects of your account health is a great way to not only make sure you’re working towards winning the Buy Box, but also earning customers’ respect and trust.

IV. Keep stock available
You can’t win the Buy Box if you’re out of stock. Keep your inventory updated and plan accordingly for things that sell quickly.

The Amazon Selling Coach can display low and out-of-stock inventory alerts, letting you know, in real time, when it’s time to restock.

Other options if you don’t win the Buy Box
Listings that do not win the Buy Box are eligible for placement in the More Buying Choices box (but please note that Amazon doesn’t guarantee placement in either of these locations).