How to open this drainage cap?
It appears that the original knockout was removed but a safety plug/cap was put on the pipe and not removed before gluing up.  Can you get a pair needle nose pliers in that little notch and see if you pull up on it?
I would drive a screw into it, leave it and tie a string to the screw. Take a Dremel or some other rotary tool and cut around the perimeter , or a razor knife or a small chisel and hammer.
The screw/string will keep the cut out part from falling down the drain.