Notice the three features added.
Convert cylinder to sheet metal solidworks.
Here is your sheet metal box.
Sheet metal convert and flat pattern suppressed.
However sheet metal parts created with the lofted bends feature avoid this limitation and unfold appropriately.
3 when all edges have been added to the two groups hit the green check.
2 solidworks will pick the rip edges for you by process of elimination.
If you cannot insert an axis then the model is not an exact cylinder and cannot be unfolded.
This often means making your first feature a base flange but it can also make use of the convert to sheet metal command if your part is imported into solidworks or if the shape is easier to model as solid no need to use the shell command but more about that in another tech tip.