The area behind the green that is not covered with turf is considered penalty area marked as red. If the penalty area is not marked with red paint the player is to use the natural margin of where the grass goes from green grass to brown or green grass to dirt or where the mowers do not cut. Your most reasonable option is to drop withing two club lengths of the entry point but no closer to the hole. This will take you left or right of you point of entry. A full explanation of your options for relief are found How to take Relief from a Red Penalty Area under the Rules of Play section of the website.