2D Nesting will not nest a sheetmetal file.
Solution
This can happen if the Layer/Color assignment options that are located in the lower left window pane are not selected. This must be set to cut so that the boundaries can be created.
Note: Flat patterns must also exist in a sheetmetal file in order for the parts to be nested.
Notes and References
Hardware/Software Configuration
Platform: INTEL
OS: windows
OS Version: 10_1809
Product: SOLID_EDGE
Application: PART/SHEETMETAL
Version: V221.0
Function: FILE
Ref: 001-10028996