Yeah, the timing is a bit tight to make the piano sound good whenever a note is triggered. One thing that would improve the experience greatly would be some visual feedback on clicks that aren’t timed properly (maybe “Early!” and “Late!” pop ups or just a different, sad particle effect), so it’s clear that the input isn’t being ignored. Or being able to tell when the input is actually ignored by mistake.
You said you were able to unlock all of the colors eventually, so at least it seems that clicks weren’t entirely broken for your browser type or something?