For words that have only 3 letters, there isn't any 4th letter. Such as with the word “add”, there isn't any additional letters Later on and therefore needs to be the word "incorporate". The word “addict” is inside the BIP39 wordlist however you would've utilised “addi” for “addict”. Mnemonic words https://bookmarkzap.com/story18705408/considerations-to-know-about-bip39-seed-phrase-generator