For words that have only three letters, there is absolutely no 4th letter. Such as with the word “insert”, there aren't any much more letters Later on and thus needs to be the word "incorporate". The word “addict” is inside the BIP39 wordlist however , you might have used “addi” https://allkindsofsocial.com/story4133772/bip39-phrase-generator-can-be-fun-for-anyone