The error looks like one you would get if you tried to add a library that is already know to calibre. If you restored the zip file to a different directory, it would probably work.
For the "zero books" problem, there is a FAQ
https://manual.calibre-ebook.com/faq...libre-is-blank. Otherwise, check that you unpacked the file properly. I have tendency to add an extra directory level when packing/unpacking things.