The default UserPlugins directory is one of the following, depending on your operating system:
However, if you did a portable install of REAPER, the UserPlugins directory will be different. You can locate the directory if you're not sure where it is.
To locate the REAPER UserPlugins directory:
In the menu bar, click Options > Show REAPER resource path in explorer/finder. An explorer/finder window opens to the location of the UserPlugins directory.