For words that only have 3 letters, there is no 4th letter. Such as with the word “incorporate”, there are no extra letters Later on and therefore needs to be the word "insert". The word “addict” is within the BIP39 wordlist however , you might have applied “addi” for “addict”. https://my-social-box.com/story4260240/new-step-by-step-map-for-bip39-english-words