NX Cannot select a datum to make a sketch line co-linear

Solution

Cannot select a datum to make a line co-linear to in Sketcher.

Notes and References

The reason that the datum plane is not selectable for constraint creation is that this sketch already has positioning dimensions. Once a sketch has positioning dimensions, it is not allowed to have constraints that reference external geometries such as datum plane/axes, external curves and edges. Deleting all the positioning dimensions will allow the datum plane to be selectable for constraint creation.  

Hardware/Software Configuration

Platform: na
OS: n/a
OS Version: n/a
Product: NX
Application: DESIGN
Version: V1957
Function: SKETCHER

Ref: 002-8569339