For words that have only 3 letters, there isn't a 4th letter. For example with the word “add”, there isn't any much more letters afterwards and thus needs to be the word "add". The word “addict” is in the BIP39 wordlist however , you might have used “addi” for “addict”. https://modernbookmarks.com/story18666364/top-latest-five-bip39-seed-phrase-mnemonics-generator-urban-news