You must have used the Zotero plugin to enter at least one in-text or numbered citation before creating a bibliography.

Place your cursor where you want your bibliography. Then select the Zotero tab within Google Docs and click 'Add/edit bibliography.'

A Google Document, displaying the Zotero menu, with the option "Add/Edit Bibliography" highlighted.

Your bibliography should immediately appear.

A Google Document, displaying an alphabetically ordered list of references.

This bibliography should automatically update if you make changes to your citations within the document. However, if you notice that it is not updating, you can force an update by going to the Zotero tab and clicking 'Refresh.'

A Google Document, with a purple circle pointing to the Zotero menu, with the option "Refresh" highlighted.