Notice the three features added.
Convert cylinder to sheet metal solidworks.
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.
If you cannot insert an axis then the model is not an exact cylinder and cannot be unfolded.
When dealing with sheet metal parts a best practice is to define it as a sheet metal part sooner rather than later.
If you can insert an axis the model is an exact cylinder.
Here is your sheet metal box.
Sheet metal convert and flat pattern suppressed.