For words that only have 3 letters, there's no 4th letter. For instance with the word “add”, there won't be any additional letters afterwards and so need to be the word "insert". The word “addict” is in the BIP39 wordlist however you would have used “addi” for “addict”. You need https://social40.com/story4297808/an-unbiased-view-of-bip39-seed-phrase-github