cats poop outside the litterbox for many reasons;
Here are some things to remember
1-Cats like privacy. Keep the litterbox in a private area, away from food.
2-Is your cat spayed/neutered. 'Fixed' cats poop less often outside the box.
3-Try changing the litter to a different brand. Some cats are finnicky when it comes to litter.
4- Be sure to change the litter COMPLETELY once a week. A cat's nose is about 30 times more sensitive than a human nose. Even if the litterbox looks and smells clean to YOU, it might not be appealing to a cat if the litter is old.
Hope that helps,