Erase is not an option in Windchill. This is a Creo function to remove any models or drawings from the active Creo session.
If you are trying to move the files around in your project, just use the Cut/Paste method described in the discussion titled "Move Command".
If you have a CAD file you don't want anymore, but Windchill will not let you remove it becuase it is related to a previous iteration of a design you created, I would suggest creating a folder named Recycle Bin and put it at the root level of your project. If you have things you want to get rid of, just put them in the Recyle Bin folder.
There is a more sophisticated means of purging the older iterations to remove the dependancy, but I cannot give you clcear guidance on that one without knowing the exact scenario. I don't want you to lose any data experimenting with this one becuase there is no Undo and there is no restore from backup on this system. If you purge or remove files, they are gone forever.