Sometimes the virtual piano fails to load throwing an error window instead. This is the case, for example, when we update some of the core files of our virtual piano app but your browser still tries to use old files stored in its cache which are no longer compatible with the new environment.
In order to fix this problem try reloading the page (Cmd + R for Mac, Ctrl + R for PC). If that does not help, you might need to go one step further and clear the browsing data (in Chrome, you can do this by pressing Ctrl + Shift + Del).