Hi Roh Mih,
Welcome to the Board!
I think this particular forum is supposed to be for people offering to be teachers or looking for teachers. Questions about particular postures are usually posted on another forum.
Just to answer your question, I would suggest that you do hamstring stretches. If you are dedicated, you can make substantial progress and probably get to the point where you can do the posture comfortably.
Some people, because of age, physical limitations, or lack of any serious martial ambition, simply slap their knee, rather than their toes. Yang Style tries to give everyone a way to practice, and so slapping your knee is acceptable in this circumstance.