For words that only have three letters, there's no 4th letter. For example with the word “insert”, there aren't any more letters Later on and thus need to be the word "incorporate". The word “addict” is inside the BIP39 wordlist however , you might have used “addi” for “addict”. Meticulously https://bookmarkfriend.com/story18876110/the-definitive-guide-to-bip39-seed-phrase