Draw (with a ruler) a long skinny rectangle, say base 6, height 1. Now draw the diagonals. You will see that the two diagonals definitely do not meet at right angles, not even close!
It turns out that a parallelogram has its diagonals meeting at right angles if and only if the parallelogram is a rhombus (all sides equal). Note that a square is a special case of a rhombus.