Use Developer Tools (Ctl-Shift-I in Windows/Chrome, Cmd-Opt-J in Mac/Chrome, F12 in IE/Edge, Ctrl-Shift-J in Windows/Safari, Ctrl-Opt-J in Mac/Safari) to view errors via the browser console if you aren’t getting sufficient information from PsychoPy. You can add print(var) (which translates to console.log(var); ) to check the value of a variable var at a particular point. N.B. There’s nothing to stop your participants doing this which may give them access to trial variables you may be loading from Excel and allow cheating on experiments with a performance based reward.