1-4 Player Expansion for Undaunted: Normandy and/or Undaunted: North Africa, which are both 2-Player. You need one of them to play Undaunted: Reinforcements. Available in about a week, at many stores (Amazon and even Walmart), but only $28.00 (~€24.8/C$35.4/£21.1) at Miniature Market.
Image Source: BGG