In the featuremanager design tree select the part and click flatten sheet metal toolbar.
Solidworks sheet metal flatten cylinder.
In either the featuremanager design tree or the cut list right click the flat pattern feature for the part and click unsuppress.
To flatten a body do one.
Flatten surface is only available in solidworks premium and is really the last chance for a part to be flattened from a 3d shape in automated way.
In the cut list right click the body and click flatten.
A cut like this will not allow the perpendicular edge it creates a slight knife edge.
I have a rectangular piece of steel 25 692 l 1 600 h 048 t that i want to bend fully to create a cylinder.
It is designed for materials like cloth or leather but can be used for metal if there will be forming involved.