@DaltonST: thanks for the tip. Firefox does not let me install "Zoteroexport" because it's not signed. I have tried with "Zotero autoexport" to export in csv and import it in calibre but that didn't work for me either.
I have also tried CalibreImportRDF but I get errors.
I have found some previous posts related to my problem:
https://www.mobileread.com/forums/sho...d.php?t=108952
https://forums.zotero.org/discussion...ibliographies/
Glad to see it's not only me! The posts are from 2010 and 2013.