I have an error when I try to get the DLC code

Can you see the DLC in your itch.io library? https://itch.io/my-collections. 

If you don't, that's because you need to buy the DLC first to be able to activate it.

I still have an error even if the DLC is in my library

We are very sorry for your issue. Sometimes, it can take up to 24 hours on itch until the API key works. We think it's due to paiement processing time. 

All we can say is trying again the next day, we know it's a pain, but unfortunately, we can't do anything since it's an Itch issue. 😢