Gradient Tool

I am trying to fill a rectangle with red to black using the gradient tool, to create a lower third.  I am guessing their are many better ways of doing this, and if anyone has ideas for me I would love to hear them!  That being said, after I create a new rectangle and then try to use the gradient tool it says "could not use the gradient tool because the layer is not directly editable"  or something of the sort.  HELP!!!

Thank you from a newby