SOLVED: on Mac OS Sierra (not High Sierra), the System Preferences' "Gatekeeper" no longer allows opening apps from "Anywhere" (only apps from Mac App Store and identified developers).

I tried control-clicking to open the app - - as one online suggestion recommended - - to no avail.

Finally using the Terminal command line at this link allowed me to open the MacBook Pro to downloading from "Anywhere"
http://osxdaily.com/2016/09/27/allow-apps-from-anywhere-macos-gatekeeper/