Going by the error you’re receiving, it could either by a typo in your handle_event/3 function name (name mismatch between your .leex and .ex files).
Another possibility, might be that you are not defining your handle_event/3 function in the correct file. For instance, is it defined in your index.ex file or your component file? My initial guess is that it is trying to look in your index.ex file or equivalent and is not finding the handle_event/3 function because it has been defined in your component (when the actual click of the button occurs in your index.ex).
So, I would check where your button click occurs and check that you have defined your handle_event/3 function there and that there aren’t any typos. Typically, the error messages are spot on.
