For words that only have 3 letters, there is no 4th letter. For example with the word “insert”, there aren't any more letters Later on and thus needs to be the word "insert". The word “addict” is inside the BIP39 wordlist but you might have utilized “addi” for “addict”. You https://thesocialroi.com/story8582797/bip39-seed-phrase-fundamentals-explained