For words that only have three letters, there is not any 4th letter. Such as with the word “add”, there isn't any a lot more letters Later on and thus must be the word "insert". The word “addict” is within the BIP39 wordlist however you would've utilised “addi” for “addict”. https://bookmarkingbay.com/story18848143/invalid-bip39-phrase-things-to-know-before-you-buy