Apple has discontinued support for full-featured Safari extensions in Safari 12. Safari 11 and earlier no longer receive security updates and are no longer safe to use.
To continue using Zotero with Safari 12, install the Zotero Bookmarklet. The bookmarklet will allow you to save items to your online library, and Zotero will then sync those items down to your computer. We'll soon be releasing an updated version of the Zotero Bookmarklet that will save all webpages (rather than just those with high-quality metadata), along with other improvements.
Alternatively, for the best Zotero experience, we recommend using the Zotero Connector for Firefox or Chrome. Firefox and Chrome provide powerful extension frameworks that allow us to offer advanced functionality such as automatic proxy redirection and automatic RIS/BibTeX interception, features that weren't possible in Safari even before Apple further restricted what Safari extensions can do.