For words that only have 3 letters, there isn't any 4th letter. For example with the word “add”, there isn't any more letters Later on and so need to be the word "incorporate". The word “addict” is in the BIP39 wordlist however , you might have employed “addi” for “addict”. https://gatherbookmarks.com/story19574030/the-definitive-guide-to-bip39-seed-phrase