The answer is 64.
The series is a list of prime numbers, except for 64, which is a composite number.
A prime number is a natural number greater than 1 that is not a product of two smaller natural numbers. A composite number is a natural number greater than 1 that is not prime.
The first few prime numbers are 2, 3, 5, 7, 11, 13, 17, 19, 23, and 29.
The next few composite numbers are 4, 6, 8, 9, 10, 12, 14, 15, 16, 18, 20, 21, 22, 24, 25, 26, 27, 28, 30, 32, 33, 34, 35, 36, 38, 39, 40, 41, 42, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, and 100.