For words that have only 3 letters, there is no 4th letter. For example with the word “insert”, there won't be any extra letters afterwards and thus needs to be the word "add". The word “addict” is during the BIP39 wordlist however , you would have utilized “addi” for “addict”. https://sociallweb.com/story5080890/examine-this-report-on-bip39-mnemonic-phrase-tool