For words that have only three letters, there is absolutely no 4th letter. Such as with the word “insert”, there aren't any additional letters afterwards and thus needs to be the word "increase". The word “addict” is while in the BIP39 wordlist but you might have employed “addi” for “addict”. https://tripsbookmarks.com/story18863009/the-ultimate-guide-to-bip39-seed-phrase-mnemonics-generator