This plugin uses a specific extension of Bukkit called CraftBukkit which is no longer visible to the public, but still exists within the code. Using the hidden extension is essential to how this plugin works, but this requires specific version references to work. So any previous versions of the plugin will only work with the specific Minecraft versions that they were released for. So this release is version 2.8 of OnePlayerSleep for Bukkit 1.16.2

EDIT: This version also works with 1.16.3