In that. The Septuagint reads, "For thou must keep the commandments of Jehovah thy God which", instead of "In that". So that this clause apparently was in the Manuscript from which the Septuagint was translated.
Related Commentaries of Deuteronomy 30:16
Deuteronomy 30:16
16 In that I command thee this day to love the LORD thy God, to walk in his ways, and to keep his commandments and his statutes and his judgments, that thou mayest live and multiply: and the LORD thy God shall bless thee in the land whither thou goest to possess it.