It might be unusual for a 40-day-old kitten not to poop. Kittens usually have regular bowel movements. Here are some possible reasons why a kitten might not poop, and some suggestions:
Dietary issues: Ensure your kitten has enough water and age-appropriate food. Sometimes, an improper diet can lead to digestive problems.
Insufficient exercise: Kittens may need some exercise to promote bowel movements. You can try using toys to encourage it to move around.
Intestinal problems: The kitten may experience intestinal blockages or other issues. If the condition persists or other unusual symptoms appear, such as vomiting or loss of appetite, it is best to take it to a veterinarian.
Environmental stress: New environments or unfamiliar things can stress kittens and affect their bowel movements. Try to provide them with a quiet and comfortable environment.
