You would get that if the Zotero Connector couldn't communicate with the Zotero app and you gave it permission to save to the online library (and maybe something is then wrong with saving to the online library, but there's no reason to be doing that if you have the Zotero app on this computer).
Restart your browser, and if the Zotero Connector asks if Zotero is running, follow the linked troubleshooting steps instead:
Restart your browser, and if the Zotero Connector asks if Zotero is running, follow the linked troubleshooting steps instead:
https://www.zotero.org/support/kb/connector_zotero_unavailable