If you are installing Sunday Keys 2021 for Ableton Live and you've already downloaded and installed Sforzando, but are receiving a VST2 error, Ableton Live is just having trouble locating it, and you can correct this with just a couple steps:
First, make sure you restarted your computer after downloading and installing Sforzando.
Then, in the Ableton Live preferences, go to the "Plugins" tab Ensure "Use VST Plug-In Custom Folder" is ON, and click "Browse" to choose the folder where all your VST plugins are. After that, click "Rescan" to make sure Ableton Live has refreshed the list of installed plugins.
On Windows, VST plugins should install to "C:\VST Plugins" by default.