For words that have only 3 letters, there's no 4th letter. For instance with the word “insert”, there won't be any much more letters afterwards and therefore need to be the word "add". The word “addict” is inside the BIP39 wordlist however, you might have applied “addi” for “addict”. Mnemonic https://e-bookmarks.com/story5250901/fascination-about-invalid-bip39-phrase