Thanks you for wanting to give it a try, and also to explain the issue clearly without being agressive, that really nice to see.

Klevernotes is a new project (± 1 year old) and doesn't have a package right now (working on flatpak). So it's pretty normal to not find it in Discover nor apt.

You guessed it right, you need to compile the project to test it.

I can try to guide you on how to use kde-src build to build klevernotes, but that might be a bit overkill since it will build every dependencies (useful for devs, a bit less for user). Or I can let you know when the flatpak is ready.