You can't model a rectangle, but you can make a 2D sketch of a rectangle :). If you're looking to model a prism, that can be done as well with a few extra steps. This will walk you through how to make the rectangle.
in the feature manager tree on the left side, you will see 3 planes (top, side, and front) right-click on one of them and click on the pencil icon with the blue line (see the first screenshot attached).
Then click on the rectangle tool (second screenshot attached)
Then in the main sketch area, click and drag to make a rectangle. (third screenshot attached)
Finally, click on the exit sketch button (fourth screenshot attached)