Launcher is asking for my registration code again and stopping me from logging on.

It is asking for a 20 digit product code, I went back to my email and found the serial#/product code and it is not 20 digits, so it doesn't fit....

If it isn't one thing it's another

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.

I only have a early access code :( in my code checker.

Edit: Also the only code I have is 20 chars long.

THIS WAS helpful mine was in junk mail as well