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It is not possible to select a datum plane from the sketch Datum CSYS to
dimension from.
Solution
Set the selection dropdown from Within Active Sketch Only to Within Work Part.
Now the datum is selectable.
Notes and References
Hardware/Software Configuration
Platform: na
OS: n/a
OS Version: n/a
Product: NX
Application: DESIGN
Version: V1847
Function: SKETCHER
Ref: 001-6874169