Along the top of the screen, you'll see a few options, one of which is Add Line.
Clicking on this option will prompt you to select a page to add the line. Once added, you can adjust the Color, Thickness, Pattern (Solid, Dashs or Dots), and the Caps (Straight or Rounded).
In addition, you can click and drag on the end points to rotate the line or change the length. To remove a line, simply click on the Trash Can icon in the lower corner.