Not a kata issue.

Description I its think of this because kata 5kyu is. Otherwise difficult it seem does not

Your solution is wrong and fails with inputs like 8675309 it returns true instead of false.

8675309 ==> [8, 6*, 7, 5*, 3, 0*, 9] ==> [8, 12, 7, 10, 3, 0, 9] ==> [8, 3, 7, 1, 3, 0, 9]
[8, 3, 7, 1, 3, 0, 9] ==> [8, 3, 7, 1, 3, 0, 9] = 31
31 (modulus) 10 ==> 1 , which is not equal to 0, so this is not a valid credit card number

Not a kata issue.

If a resulting number is greater than 9, replace it with the sum of its own digits (which is the same as subtracting 9 from it)

You're not doing that right.

Everything is working on my end, and I am passing the test. But when I attempt, it is saying expected true to equal false and vice versa if I change.

Should the output be string type? How to include that n.