I will summarize here what others have already mentioned and some more.1) Undo has been intentionally disabled for online contests2) You can use "Clone" button to do your trials.3) Additionally, you can also use other colored lines for trials, Note: The solution check accepts all colors4) If you want to learn other shortcuts and other modes available, you can always refer to the readme document - https://github.com/swaroopg92/penpa-edit/blob/master/README.md [Also accessible through "I" button on top of the penpa_PLUS_ page -> Tool Instructions]I hope this helps.