It depends what you are trying to do. If it's simply to create collections within Calibre, then that facility exists via custom columns & tags. Tags can be nested.
If you're wanting to copy the collections to a Kindle, try Kindle Collections plus Librarian Sync -
https://www.mobileread.com/forums/sh...librarian+sync.
AFAIK, there's no longer the ability to create nested collections within Librarian Sync.
Collections Manager itself is (I
think) reitred, but the thread has some good stuff on collections generally.