You can buy DJI Care directly from DJI here if it has never been purchased for that drone. If the previous owner already purchased DJI Care, you'll be able to use the remaining coverage until the expiration date (one year after the purchase date). After the DJI Care protection plan expires, it cannot be renewed.
For drones that have been activated more than 48 hours ago, DJI requires the owner to create a video to show all is in working order before DJI Care can be purchased. You can find more details about that process here.