If you’re trying to copy and paste into the two staves of the piano, try copying and pasting one staff at a time: Dorico copies and pastes by instrument, rather than by staff, so if you copy music from e.g. separate soprano and alto instruments and paste them to the piano, you’ll find that the soprano music goes into the right hand staff of the piano, but the alto music goes into the instrument below the piano (if there is one). To paste e.g. the alto music into the left-hand staff of the piano, make a selection that contains only that single staff, and then paste it onto the left-hand staff directly.