Microsoft Office for Mac 2011 has officially reached end of life, and Microsoft is no longer supporting or updating it.

Furthermore, Microsoft has stated that Office 2011 apps have not been tested with Mac OS 10.13, and that users should upgrade to Office 2016 for High Sierra compatibility.  While Office 2011 apps may appear functional to one degree or another after upgrading to Mac OS 10.13, it is not a supported configuration, and should be considered use-at-your-own-risk.