For words that only have 3 letters, there is no 4th letter. One example is with the word “insert”, there isn't any more letters Later on and thus should be the word "include". The word “addict” is within the BIP39 wordlist however , you would've utilised “addi” for “addict”. Mnemonic https://ledbookmark.com/story4449919/invalid-bip39-phrase-things-to-know-before-you-buy