For words that have only 3 letters, there is no 4th letter. One example is with the word “insert”, there are no additional letters afterwards and therefore must be the word "increase". The word “addict” is inside the BIP39 wordlist however , you might have applied “addi” for “addict”. You https://bookmarkextent.com/story20583816/invalid-bip39-phrase-things-to-know-before-you-buy