I am trying to fix the values of some angles and points. I am also trying to make an existing line stay parallel to another line. How do I define 2d constraints in Elements? How do I lock a dimension the way you can in Creo?
The general philosophy of Creo Elements/Direct is that 2D geometry is a tool for creating and modifying 3D, and that 2D is not needed any more once the 3D is created or modified. Therefore, there has been no reason to put constraints on 2D geometry, only on 3D geometry.
In version 19 there is the capability to attach 3D dimensions to 2D elements. Those dimensions can be used to document your sketch, and they can be used to drive changes to your 2D geometry.