Odd. When I do a portable install to a new location, and even when I then reinstall to that same new location, I don't get the error. (Even after running the new install and using the notation editor, which is what I assume the font is for...?)
But it consistently gives that error when installing over my active R5 install folder. When I try to delete Reavura.otf from that folder, I get an error that the file is "open in System."
But what else would be using it if Reaper isn't running at the time?