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Hello. How can I score this situation?
After the ball being batted the runner on the first base attempts to advance to the second base. SS stands on the second base trying to catch the batted ball. To reach and occupy the first base the runner knocks the SS off his feet so that he is not able to catch the ball properly. So the ball is not caught and the batter-runner reaches the third base.

I shall let others determine what the rules are here, but I hope this SS learns not to block the base path without the ball in his/her glove. There are plenty of ways to make that play without standing on the bag. (I assume this would have been a force out and not requiring a tag.) Otherwise they will rightly end up on their keister.

No force out. SS did not catch the ball. He was looking at the ball, and the runner did not see.