You cannot see Constraints in the graphics-area.

To DELETE a Constraint:

1.SHIFT + Click a sketch-element or a Point* that has a constraint you want to delete.

All of the Constraints that apply to the sketch-element or Point* are now in the Selection-Window

In the Selection-Window:

2.Right-click the Constraint you want to delete

3.Click Delete element in the shortcut menu

* Point, start-Point, end-Point, or center-Point

Add MERGE-POINTS moves two Points* together, and deletes one of the Points.

Add Coincident Constraint is between two Points*. The command does not delete one of the Points*.

You can delete a Coincident constraint- see How to delete a constraint.

Usually, add a Coincident constraint for construction geometry.

* Points, start-Points, end-Points, center-Points.

You cannot OVER-constrain a sketch-element.

For example, a Point* has only two degrees-of-freedom.

If you:

1.Add a Coincident constraint between a Point* and a Line

You cannot:

2.Also add a Coincident constraint between the same Point* and the start-Point or end-Point of the same Line.

Why?

You cannot remove three degrees-of-freedom from a Point.

You remove one degree-of-freedom from the Point. when you do '1'.

You attempt to remove two more degrees-of-freedom from the Point in '2'.

What do I need to do?

1.Delete the Coincident constraint between the Point* and the Line - see How to Delete Constraint  

2.Add the Coincident constraint between the Point* and the start-Point or end-Point of the Line.