Moses gives a long, boring, final speech in the last few chapters of Deuteronomy. Just before the speech, he listed the blessings of obeying the all of the laws in the Torah and all of the curses for not obeying them. So when Moses says that "the word" is "in thy heart" in Dt 30:14, he's talking about the laws of the Old Testament.
But the word is very nigh unto thee, in thy mouth, and in thy heart, that thou mayest do it. Deuteronomy 30:14
Paul misquotes Dt 30:14 by claiming that "the word" in Dt 30:14 refers not to the Old Testament law (as it clearly does), but to "the word of faith" that Paul is preaching.
That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised him from the dead, thou shalt be saved.
But what saith it? The word is nigh thee, even in thy mouth, and in thy heart: that is, the word of faith, which we preach. Romans 10:7-8