Finally creating it as a loft generated the correct flat pattern. Most attempts ended with a flat pattern that was simply a rectangle, this was clearly not correct. Last night I wanted to create a sheet metal cone that had a 16' OD on one end, a 12' OD on the other. The program needs something fixed or it has no idea what the reference 'zero' is. Solid models converted to sheet metal need a fixed edge/surface as a reference for all other features to 'unbend' around, sort of like it's an axis for rotation/unbending. The sheet metal module operates a bit differently than solid modeling. Very often a part created in solid modeling may convert to sheet metal but not be able to generate a flat pattern.

Expand the feature (+) right click on the mouse on the 'Flat Pattern' (try one or the other) will give you the option to un-suppress it and lay it flat. The 'Flat Pattern' is the bottom feature in the model tree and it's suppressed. With sheet metal parts there's a 'Flat Pattern' feature that's created automatically but it greatly depends upon how the part was created. I think the latest version I've used is 2019.

Mastercam 2018 for solidworks