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In your 4th code-box the first row is missing, i.e.:

horizontalLetter2D[lt_, sc_, dx_, dy_] :=

and in the last code-box you used different variable names then in the code-box before (interStar instead of xmasStarPoints, anaStar instead of anaXmasStarPoints). It should read:

Graphics3D[{mirror, {{FaceForm[Green], EdgeForm[AbsoluteThickness[1]],
     Polygon@
      Flatten[xmasTreePoints, 1] /. {x_?NumericQ, y_} -> {0, -x, -y}, 
    EdgeForm[AbsoluteThickness[3]], 
    Polygon@Flatten[anaXmasTreePoints, 1] /. {x_?NumericQ, y_} -> {-x,
        y, 2}}}, {{FaceForm[Orange], EdgeForm[AbsoluteThickness[1]], 
    Polygon@Flatten[xmasStarPoints, 1] /. {x_?NumericQ, 
       y_} -> {0, -x, -y}, EdgeForm[AbsoluteThickness[3]], 
    Polygon@Flatten[anaXmasStarPoints, 1] /. {x_?NumericQ, y_} -> {-x,
        y, -2}}}, {AbsoluteThickness[3], 
   Line[{{-6, -5, -2}, {-6, 6, -2}, {6, 
      6, -2}, {7, -5, -2}, {-6, -5, -2}}], 
   Line[{{-6, -5, 2}, {-6, 6, 2}, {6, 6, 2}, {7, -5, 2}, {-6, -5, 
      2}}], AbsoluteThickness[2], Blue, 
   Line[{{0, 1, 0}, {0, 6, 0}}]}}, Boxed -> False, ImageSize -> 250]

Then everything is working fine:)

Fantastic work Erik. Thank you for sharing.