2 solidworks will pick the rip edges for you by process of elimination.
Convert cylinder to sheet metal solidworks.
If you cannot insert an axis then the model is not an exact cylinder and cannot be unfolded.
As a test try to insert an axis on the cylindrical face.
Sheet metal convert and flat pattern suppressed.
When dealing with sheet metal parts a best practice is to define it as a sheet metal part sooner rather than later.
Notice the three features added.
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.