Sometimes when cats poop outside their litter box it could be because of the litter box not being cleaned often enough and the poop and pee builds up in the litter box and the cat decides not to use it.
Cats love to use a clean spot to poop or pee and if the litter box is too full or not smelling clean enough then they may decide to poop on the floor or pee on the floor instead.
If your cats litter box is clean then it may be a health issue that the cat is pooping or even peeing on the floor outside the litter box.
Sometimes cats will pee or poop on the floor and outside of litter boxes due to lower urinary tract disease (FLUTD), bacterial cystitis, kidney failure, liver disease, or diabetes.
When a cat poops or pees on the floor make sure to clean the area with an enzyme cleaner to get rid of the scent so that they do not poop on the floor in that area again.
If your cat is pooping on the floor or outside the litter box try to catch then and gently tell them no that they need to use the litter box.
Or even train the cat to go outside like I did with one of my cats.
I had one cat that refused to use a litter box and then he learned to go outside when he needed to pee or poop and he would even let me know at night by waking me up and then I would let him out for a bit and then he would come back in after he was done pooping or peeing.