The mistake is in Step #4. The current instruction to draw a line that passes through points A, C, and B does not actually provide a clear bisector; instead, it implies drawing a line directly through the segment, which does not define a bisector.
The correct instruction should emphasize that the line drawn only needs to pass through point C, since point C is the point of intersection from the folds which will bisect segment AB.
So, the best option is:
Step #4 has a mistake. It should say: “Use a straight edge to draw a line that passes through point C (but not points A and B). This line is a bisector of AB¯¯¯¯¯¯¯¯.”