For words that have only three letters, there is not any 4th letter. Such as with the word “add”, there are no extra letters Later on and thus needs to be the word "insert". The word “addict” is within the BIP39 wordlist but you would've applied “addi” for “addict”. - https://baidubookmark.com/story18892890/the-fact-about-valid-bip39-seed-phrase-that-no-one-is-suggesting