For words that have only three letters, there is absolutely no 4th letter. Such as with the word “insert”, there won't be any more letters afterwards and thus needs to be the word "include". The word “addict” is in the BIP39 wordlist however you would've used “addi” for “addict”. You https://orangebookmarks.com/story19531212/bip39-phrase-generator-can-be-fun-for-anyone