For words that have only 3 letters, there's no 4th letter. For instance with the word “increase”, there won't be any much more letters afterwards and so has to be the word "incorporate". The word “addict” is inside the BIP39 wordlist but you might have employed “addi” for “addict”. bip39-phrase.com https://writeablog.net/ipennywpzc/h1-b-opening-the-power-of-bip39-exactly-how-to-produce-and-use-mnemonic