[Solved]-("svg:rect") element does not appear on the screen when run-d3.js

You're assigning the rectangle's attributes to the group (i.e, svg:g) beneath it. In the line below, remove the .append("svg:g") and it should work.

square.enter().append('svg:rect').append("svg:g")

Also, you're setting opacity to zero:

.attr('opacity', 0)

This makes your rectangle invisible. Set it to 1 instead for full visibility, or some number in between for a translucent fill effect.