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If you look in the lower left of the screen and you have the line selected it will show 1 Line on 1 Layer.

Select the Shape Tool (F4) and either click and drag from one end point to the other end point. The cursor will change to display a + and the line will become a shape and fill in.

If your start and end point overlap, drag the end point away a small distance then back over the other point. Again the cursor will display a + and the new shape will fill.