For words that have only 3 letters, there isn't any 4th letter. For instance with the word “insert”, there are no far more letters Later on and therefore need to be the word "incorporate". The word “addict” is during the BIP39 wordlist however you might have employed “addi” for “addict”. https://freebookmarkpost.com/story18830746/the-ultimate-guide-to-bip39-seed-phrase-mnemonics-generator