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".
Solution
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
Hardware/Software Configuration
Platform: INTEL
OS: window
OS Version: 764SP1
Product: NX
Application: NX_SHEET_METAL
Version: V8.5.3
Function: FLAT_SOLID
Ref: 002-7004433