For words that only have three letters, there's no 4th letter. Such as with the word “insert”, there are no a lot more letters Later on and thus have to be the word "incorporate". The word “addict” is within the BIP39 wordlist however you would have utilised “addi” for “addict”. https://health-lists.com/story19444689/helping-the-others-realize-the-advantages-of-bip39-mnemonic-phrase-tool