Reference lines are different from regular lines. They don't have start and end points. They will originate from a single point and then draw out from there.

Select a starting point to begin.

Line Direction

Next you will need to select the direction of the line.

If you choose Horizontal or Vertical, a straight reference line will be drawn horizontally or vertically originating from the selected point that spans the canvas.

If you choose On Vector, you will be asked to select a point to determine the vector.

Now a line will be drawn that passes through the two points spanning the canvas width.