If you want to create a copy of an Assembly, you also need to copy all the Sub Assembly and Part files (Components) that go with it.
And that’s not all. If you are copying files within a Project, you will also need to re-name all the components, otherwise Inventor may find the wrong version of the component that you intended to be used in the new Assembly.
And that’s not all! An Assembly file contains is a list of Hyperlinks to other Assembly and part files. If you copy, move and rename all the sub components of an Assembly, you will also need to repair all those Internal Hyperlinks.