As with all citation managers, you need to double check your citation information to make sure it is correct and conforms with the standard that you are using. It is suggested that you edit the citation information before creating your bibliography; otherwise, you will need to edit the bibliography in your document.
Reference items need to be in a Collection in order to proceed. If necessary, Create a Collection and add Reference items.
Even after formatting - you still need to check to make sure that the citation information is correct. There are several errors in the example below.
If you are using the Zotero plugin with GoogleDocs, Microsoft word, or LibreOffice, you must have references in your document already. Then you can use the Add/Edit Bibliography feature.