For words that have only three letters, there is no 4th letter. Such as with the word “insert”, there won't be any a lot more letters Later on and therefore has to be the word "incorporate". The word “addict” is in the BIP39 wordlist however, you would have utilised “addi” https://listbell.com/story8535400/considerations-to-know-about-bip39-seed-phrase-generator