Hi. I have the exact same issue here. I had an older version of Maps for Office on my old laptop. Working just fine with no such warnings. Lately it has ben reminding me of an update pending, which I´ve ben ignorring since I was about to get a new laptop. Now when I finally got the new PC and installed Maps for Office with the update to latest version it gives me this warning on application startup. I'm connecting to my organization ArcGIS Online account (not an Enterprise Portal) in order to use our own basemaps and address locator.
What I do not understand is why "configuration of Portal " is mentioned when it is ArcGIS Online that I am connected to?