Thank you! My problem, however, is that the instructions on this site do not work. They say: click on Safari -> Extensions and select Zotero, but there is on Zotero option there.
A smart user helped me in the real world. For the benefit of all who have the same problem: You need to avoid the "Safari extensions" section in the pull out menu. Instead, go to "Settings", and then select "extensions", where you will find zotero. Thanks to dstillman anyway for generously offering help.
1) Do you have the Zotero app installed?
2) What macOS version?
3) Have you tried deleting Zotero from applications, restarting your Mac, and reinstalling the app?