Unfortunately S-Note has been discontinued and replaced with the horrid, bare-bones, good-for-nothing Samsung Notes.
I was able to backup the version I had on my Note 5/Tab S with Titanium backup and use that APK to side-load it onto my S8. Not sure why you can't install it (unless you're trying to install a Samsung Notes APK).
It sucks that the latest few versions of S-Note, though, removed a very good feature: convert handwriting into text. But even then it's a million times better than Samsung Notes.