Microsoft.Office.Core Reference Missing

None of the above answer helped me, i was using Visual Studio 2017. What I did is, installed Office/SharePoint Development using Visual Studio Installer.

After that, I was able to see 'office', this assembly contains Microsoft.Office.Core.

Hope this helps you.

You can add reference of Microsoft.Office.Core from COM components tab in the add reference window by adding reference of Microsoft Office 12.0 Object Library. The screen shot will shows what component you need.