Hello!
I have not tried this but maybe you can try the below code instead? 
| WTPart part = getPart("name"); | |
| SubFolder folder = getSubFolder("libraryName", "folderPath"); | |
| try{ | |
| FolderHelper.assignLocation(part, folder); | |
| }catch(Exception e){ | |
| e.printStackTrace(); | |
| } |
I know the API is deprecated in Windchill 10 but there are some other similar methods there you can use that will probably do the same.
Best regards,
Peter