When performing Insert -> Flat Pattern -> Flat Solid, using the main inside face, NX fails to create flat solid due to error "Unable to create body".
The problem happens because sheet metadata contains Closed Corners features, and the Closed Corners were not created properly.
Resolution is to edit all Close Corner features, change the Treatment option from "Circular Cutout" to "Open". Then the Flat solid will be created successfully.
Notes and References
OS Version: 764SP1