If the PS5 rejects the code, there are a couple of likely causes. The first is that you've simply entered the code incorrectly, which is very easy to do by mistake. Or alternatively, the code has already been redeemed. This can happen if a store has accidentally sold duplicates, or someone has sneakily redeemed the game without paying for it. If you think this is the case, make sure to contact whichever store supplied you with the redeemable code.