See the Chromebook support documentation for help installing Android apps on Chromebooks, and a list of supported devices.
The app only supports playback of existing scores, not score editing or creation, but you can sign-in to your Muse Score account for easy access to all your scores on Muse
The App Image has some special options in addition to Muse Score's normal command line options.
Either you can fix the association, or download and use the portable version of Muse Score instead.
Continuing you'll see If you click to continue, the setup wizard displays the terms of the free software license.
If you prefer, there is still the option to get it the traditional way via your distribution's package manager (but you may have to wait for it to get packaged by the relevant maintainer). The App Image format is a new way of packaging Linux applications.
App Images are portable - they don't have to be installed - and they run on pretty much any Linux distribution.