You can draw 2D circles and ellipses by using the Circle and Ellipses tools, respectively, in the Sketch menu.
Select Circle and then select the grid to place the center and drag your Apple Pencil until you reach the correct radius length. If you want to redefine the shape’s radius, tap on the dimension label and add a value in the numpad.
Tap Ellipse and place the center of your ellipse on the grid. Drag your Apple Pencil to create the shape. You can then define the major and minor axes by tapping on the dimension labels.