If you can insert an axis the model is an exact cylinder.
Convert cylinder to sheet metal solidworks.
Notice the three features added.
2 solidworks will pick the rip edges for you by process of elimination.
Here is your sheet metal box.
If you cannot insert an axis then the model is not an exact cylinder and cannot be unfolded.
3 when all edges have been added to the two groups hit the green check.
However sheet metal parts created with the lofted bends feature avoid this limitation and unfold appropriately.
Sheet metal convert and flat pattern suppressed.