"Sheet metal part in STP format" means that the modeling of the sheet metal part is created in other software, and then exported to STP format, all the features about the sheet metal modeling have disappeared in this export process, and then imported to UG, in the view of the UG software, this "sheet metal part" is just a solid, without any modeling features. After importing into UG, in the view of UG software, this "sheet metal part" is just a solid, without any modeling features, so it can't be expanded.

Unless you create the model in UG with UG's sheet metal commands, then you can expand it. ok ?