There is currently no "Line Tool" per se in Muse today, but there is a viable workaround. In the Control Strip at the top of the application select the Rectangle Tool, and press and drag a rectangle on the canvas:
With the rectangle selected click on the Stroke drop down window, then "break" the link constraining all of the sides of the rectangle by clicking on the icon in the center of the four values defining the stroke of the rectangle:
When you are back on the canvas in the Design view you can resize the rectangle now to be the length you'd like.
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