Run Tools/Sketch tools/Check Sketch for Features...
Select Base Extrude and click Check.
Now SW will tell you what's wrong.
The problem is most probably what Mark mentions.
You might have inadvertently drawn an extra line when you
double click after drawing many lines. This happens to me alot. You
might try dragging your mouse over each corner of the rectangle and
hitting delete to get rid of any potential entities living there
and try again.
Select Base Extrude and click Check.
Now SW will tell you what's wrong.
The problem is most probably what Mark mentions.