Click the green flag. (Note: this is a 2 player game.) The "a" key and "d" keys control the blue platform, (you have to press and hold them) and the left and right arrow keys control the red platform. (you have to press and hold them as well.)
I used the same Udemy course as my "Flappy Bird" game, so if you haven't played it, here is the link. https://scratch.mit.edu/projects/229023858/
