Does creo can finish shell commands? First,All dimensions are not allowed to modify.Second,do not modify the sequence of feature.Third,remove the red face.
On the back side at the base of the protrusion you are trying to remove the face from you need to exclude the surface there to get it to shell. The surface causes geometry problems if you don't exclude it.
Kevin's answer is perfect for your situation, and not everyone is aware of the "exclude surfaces" option. Thanks Kevin! If you do a lot of this kind of work, you might want to use another alternative approach in addition to Shell. If you copy all of your external geometry into one or more Surface features, you can then create Offset Surface features from them and use that geometry (Merging if necessary) to "cut out" your model using Edit/Solidify. This sometimes gives you more flexibility dealing with minimum radius fillets, elimination of unecessary fine detail, adding intenal geometry, etc.
Did you try the directions above? I don't have access to Creo 1 to try your part but the pictures are from using the IGES file you posted shelled in Creo Elements/Pro 5 M090. If you try to shell the part thinner than 1.93 you may have problems. From measurements you can go to about 1.7 before you start to see geometry problems. If you need to go thinner you may need to use the method David suggested of offsetting the surface and using the solidify feature.