Set the Test Duration BEFORE you click the green flag You will need to click the green flag and then click stop when you first load it. Then, set the Test Duration. (this is only for the first time) Don't be worried if the CPS score does not show while you are clicking, it is only supposed to show when your test is done.
I wanted to replicate the Clicks Per Second Test on Rapid Tables. Here's a link: https://www.rapidtables.com/tools/click-test.html Credit to @PaulRHJT for song "Elucidate"