For words that have only 3 letters, there isn't any 4th letter. For instance with the word “add”, there won't be any extra letters Later on and therefore needs to be the word "insert". The word “addict” is while in the BIP39 wordlist however , you might have employed “addi” https://altbookmark.com/story20666161/ledger-seed-phrase-bip39-no-further-a-mystery