What version of RStudio do you have? The "Save as Web Page" button seems to work with the current stable release of RStudio (v1.2.1330) and plotly (v4.9.0).
You can also use htmlwidgets::saveWidget() to save an HTML file programmatically
Thanks very much for the comment: that encouraged me to look at the html file produced by
the htmlwidget command. The file looked ok. But it was showing up as a blank page on Firefox browser. So I tried Chrome, and it now works great!!!