A common issue is confusion of certain letters and numbers. Notably "I," "l," and "1."
- Capital "I" as in India.
- Lowercase "l" as in leopard. We do not use this letter in our codes.
- The number "1." We do not use this number in our codes.
Another reason may be that you have a used textbook and the code is no longer valid. You may also purchase course access only.