The accepted solution didn't solve it for me.
Since one of the most recent updates (10.13 or 10.14) the reader view shortcut in Safari has stopped working, and now produces a high pitched short sound. The shortcut for the Show Reader menu in Safari is "Shift-Command-R" as it used to be, however now the following support article https://support.apple.com/en-ca/HT201236 lists "Shift-Command-R" as the "Open the AirDrop window" shortcut on the macOS level.
Clearly Apple has let a collision through, and then they wrongly prioritize the AirDrop shortcut even when Safari is in focus and full screen.
As a user, I personally use the Reader view on and off a multitude of times a day as I go about my browsing, and rarely every use AirDrop, not to mention need a global shortcut for it.