Missing registration code

[worldofneil]

the registration code field was blank

Sorry if you already did this, but show that to the shop. If they see that there isn't a code where one would be, they should be obligated to replace it with a copy that does have the code (of course your laws may vary). They might not be the creators of the product, but they are the ones selling it and as-is they haven't sold the product you paid for.

Because the code area is blank they can be 100% sure you aren't lying to get a 2nd copy so there should be no problem with them replacing it with another one (open it in store with them to confirm it has a code) and they can take the matter up with SE (most likely they'll return it as damaged/faulty goods).

If you didn't have the code sheet at all it would likely be trickier, but having a sheet with no code is pretty obviously "faulty".