Create table and change background color using JavaScript

My task is this: I want to create a page with the button and a horizontal table of results from 1 to 20.

When you click a button, you get a random number, display it on a page and the cell with this number should change color to green. If the number appears for the second time, make it grey.

If the cell is already grey, nothing happens. That's it this is my code plth helpm me if you can. thanks