There is a loophole when trying to install the extension for Mac (Latest macOS Sierra vb. 10.12.6 and Safari v. 11.0.1) where on the OneNote site, url https://www.onenote.com/clipper there is a "Get OneNote Web Clipper for Safari" link, which redirects you to the Apple extensions site, url https://safari-extensions.apple.com/details/?id=com.microsoft.onenote.clipper-UBF8T346G9, which it turn sends you back to the OneNote site.

Please help and,

I appreciate your decency (Microsoft support team).