For words that only have three letters, there isn't a 4th letter. As an example with the word “add”, there are no more letters Later on and thus have to be the word "insert". The word “addict” is in the BIP39 wordlist but you might have applied “addi” for “addict”. https://thebookmarkage.com/story18779203/helping-the-others-realize-the-advantages-of-bip39-mnemonic-phrase-tool