Answer:
Here you go, alter this as you see fit :)
Explanation:
array = []
cnt = 0
while cnt < 11:
  x = input("Enter a word: ")
  array.append(x)
  cnt += 1
  y = input("Add another word?(Y/n):  ")
  if y.lower() == "n":
    break
letter = input("\nChoose a letter: ")
if len(letter) != 1:
  print("Error: too many characters")
  quit()
for n in range(len(array)):
  if letter.lower() in array[n].lower():
    print(array[n], end= ",")