The Sketcher Fix Constraint locks a point onto an Object.
The “Fix a Point to an Object” attaches a vertex to an edge.
Line not Line-Segment
The Fix Constraint attaches the vertex to the Line not just the visible line segement. In the figure below the rightmost line is constrained to the leftmost, the vertex is simply not within the Line Segment displayed.
If Auto-Constraints are enabled the Fix constraint will be shown when drawing line segment. When adding the second vertex of a line segment the Fix constraint will be show when a qualified edge is under the cursor.
A note about the B-Spline. When creating the line FReeCAD will allow the attempt of fixing to the B-Spline, it will however result in an error. You will need to manually remove the numbered constraint.
If the Vertex and B-Spline are pre-selected you will receive an error dialog.