Orange juice is not a pure substance but a mixture of different pure substances. Pure substances are made of only one type or atom or molecule. Orange juice, even fresh squeezed from the orange and not reconstituted with water and concentrate, is made of of multiple different substances, including sugar, citric acid, vitamin C and water.
Elements like sodium are pure substances because their molecular structures consist of just one type of atom. Similarly, pure water that has nothing but two hydrogen atoms and one oxygen atom is a pure substance. Things like lemonade, which are a mix of water, sugar and lemon juice, are mixtures rather than pure substances.
