For words that have only 3 letters, there's no 4th letter. Such as with the word “insert”, there won't be any far more letters afterwards and therefore has to be the word "include". The word “addict” is in the BIP39 wordlist however , you might have employed “addi” for “addict”. https://bookmarkfame.com/story18890295/getting-my-bip39-words-to-numbers-to-work