A pinhole camera is a box with a very small hole in one end of it and a piece of film(screen) in the middle. The light from the object after passing through a very tiny pinhole camera hole falls on the screen.
If the screen is transparent, light will pass through the screen without forming an image.
If we choose an opaque screen, although the image will be formed, but the observer won't be able to see it from the other end. This is because an opaque screen doesn't allow light to pass through it at all.
Therefore, we need a translucent screen for light to fall on and form an image, while at the same time allowing some light to pass through it so that it's visible to the person on the other side.