A hidden single is different from a naked single. The cell may still have multiple candidate digits, but one digit has only one possible home in a row, column, or 3×3 box.
How to scan: pick a unit, choose one missing digit, and ask: “where could this digit go?” If only one empty cell can accept it, that cell is a hidden single.
This board starts with several hidden singles. Do not just look for cells with one candidate — scan rows, columns, and boxes for digits with one legal home.