There are two codes people received:

The first is the pre-order code, which was 18 characters long.
The second is the actual game code, which is 20 characters long.

Check your retailer for the second code. For example, I bought my copy on Green Man Gaming, so I checked my game Codes, and there are two there; the pre-order, and the game code.

If you never entered the game code, your pre-order code only lasts for a few days after the game went live in case you were to cancel your purchase.