How to thicken lines with line tool

Lines by themselves cannot be thickened. You should think of them as being infinitely thin.

If you want a thicker outline you need to create a 2D shape that matches the line that you want and use Fill operation.

An easy way to go from some lines to 2D shapes is to select the shape, and then use the Offset Shapes tool on the left toolbar. I’d suggest using the “Both” selection for both inside and outside offset and choosing to delete the original shape.