For words that have only 3 letters, there is no 4th letter. Such as with the word “incorporate”, there won't be any extra letters Later on and thus need to be the word "include". The word “addict” is during the BIP39 wordlist however you would've utilized “addi” for “addict”. Various https://ok-social.com/story4400683/5-essential-elements-for-bip39-explained