The reason this is failing is because (Python 3) input returns a string. To convert it to an integer, use int(some_string).
You do not typically keep track of indices manually in Python. A better way to implement such a function would be
def cat_n_times(s, n):
for i in range(n):
print(s)
text = input("What would you like the computer to repeat back to you: ")
num = int(input("How many times: ")) # Convert to an int immediately.
cat_n_times(text, num)
I changed your API above a bit. It seems to me that n should be the number of times and s should be the string.