Here is an example of a parallelogram that is not a rectangle with an area of 18 square units:
First, draw a horizontal line segment of length 6 units. Then, draw a vertical line segment of length 3 units starting from the right end of the horizontal segment. Draw another horizontal line segment of length 6 units starting from the top end of the vertical segment, parallel to the first horizontal segment. Finally, draw a vertical line segment of length 3 units starting from the left end of the second horizontal segment, parallel to the first vertical segment. This should form a parallelogram with an area of 18 square units.
The parallelogram should look something like this:
```
+---+
/ /|
+---+ |
| | |
| | +
| |/
+---+
```
The horizontal sides have a length of 6 units, and the vertical sides have a length of 3 units. The area can be calculated as base times height, which in this case is 6 units times 3 units, or 18 square units.