To learn more about these vulnerabilities, see Microsoft Common Vulnerabilities and Exposures CVE-2017-8631, CVE-2017-8632, and CVE-2017-8742. If your version of office mac is home and business then you can use this link.
Suggest you to look for the direct download links available online and use them to install office mac.
This security update resolves vulnerabilities in Microsoft Office that could allow remote code execution if a user opens a specially crafted Office file. Looks like MS has stopped the download link to office 2011 mac. It includes fixes for vulnerabilities that an attacker can use to overwrite the contents of your computer's memory with.
Which is why the company has decided to offer Mac users a free, one-month trial of the latest version of their popular Office productivity suite, Microsoft Office for Mac 2011.