Sirtel advice is 100% spot on
The GUI has Copy to Library on the context menu (add it using Preferences: Toolbars... if you need to)
The Calibre you are using needs to know BOTH Libraries.
The Usage is while viewing the FROM library.
Select the book to copy: select Copy to Library (from the ctx-menu):Select the destination Library
...delete after makes it a
Move.
There is a commandline interface method, but it is a bit more effort to use as you need to 'select first, then ...

Pick the GUI