To get familiar with this i would suggest you to check documentation about the transport case to see what it actually does:
hereTo explain it, would burst this thread.
Based on the info in docu the answer below:
The errormessage appears if you try to delete an object which was put to transport case to export it out of your system. Please access the transport case (from automic process assembly view or in Java GUI explorer), search the object and remove it from the transport by right clicking on it, and select "remove".
after this it should possible to delete the object.